[examples] Added: `shapes_rlgl_triangle` example (#5353)

* [examples] Added: `shapes_rlgl_triangle` example

* correct name

* formatting

examples/shapes/shapes_rlgl_triangle.c

@@ -0,0 +1,189 @@
+/*******************************************************************************************
+*
+*   raylib [shapes] example - rlgl triangle
+*
+*   Example complexity rating: [★★☆☆] 2/4
+*
+*   Example originally created with raylib 5.6-dev, last time updated with raylib 5.6-dev
+*
+*   Example contributed by Robin (@RobinsAviary) and reviewed by Ramon Santamaria (@raysan5)
+*
+*   Example licensed under an unmodified zlib/libpng license, which is an OSI-certified,
+*   BSD-like license that allows static linking with closed source software
+*
+*   Copyright (c) 2025-2025 Robin (@RobinsAviary)
+*
+********************************************************************************************/
+
+#include "raylib.h"
+#include "rlgl.h"
+#include "raymath.h"
+
+//------------------------------------------------------------------------------------
+// Program main entry point
+//------------------------------------------------------------------------------------
+int main(void)
+{
+    // Initialization
+    //--------------------------------------------------------------------------------------
+    const int screenWidth = 800;
+    const int screenHeight = 450;
+
+    SetConfigFlags(FLAG_MSAA_4X_HINT);
+    InitWindow(screenWidth, screenHeight, "raylib [shapes] example - rlgl triangle");
+
+    // Starting postions and rendered triangle positions
+    Vector2 startingPositions[] = {{ 400.0f, 150.0f }, { 300.0f, 300.0f }, { 500.0f, 300.0f }};
+    Vector2 trianglePositions[] = { startingPositions[0], startingPositions[1], startingPositions[2] };
+
+    // Currently selected vertex, -1 means none
+    int triangleIndex = -1;
+
+    SetTargetFPS(60);
+    //--------------------------------------------------------------------------------------
+
+    // Main game loop
+    while (!WindowShouldClose())    // Detect window close button or ESC key
+    {
+        // Update
+        //----------------------------------------------------------------------------------
+        // Reset index on release
+        if (IsMouseButtonReleased(MOUSE_BUTTON_LEFT))
+        {
+            triangleIndex = -1;
+        }
+
+        // If the user has selected a vertex, offset it by the mouse's delta this frame
+        if (triangleIndex != -1)
+        {
+            Vector2 *position = &trianglePositions[triangleIndex];
+
+            Vector2 mouseDelta = GetMouseDelta();
+            position->x += mouseDelta.x;
+            position->y += mouseDelta.y;
+        }
+
+        // Enable/disable backface culling (2-sided triangles, slower to render)
+        if (IsKeyPressed(KEY_LEFT))
+        {
+            rlEnableBackfaceCulling();
+        }
+
+        if (IsKeyPressed(KEY_RIGHT))
+        {
+            rlDisableBackfaceCulling();
+        }
+
+        // Reset triangle vertices to starting positions and reset backface culling
+        if (IsKeyPressed(KEY_R))
+        {
+            trianglePositions[0] = startingPositions[0];
+            trianglePositions[1] = startingPositions[1];
+            trianglePositions[2] = startingPositions[2];
+
+            rlEnableBackfaceCulling();
+        }
+        //----------------------------------------------------------------------------------
+
+        // Draw
+        //----------------------------------------------------------------------------------
+        BeginDrawing();
+
+        ClearBackground(RAYWHITE);
+
+        if (IsKeyDown(KEY_SPACE))
+        {
+            // Draw triangle with lines
+            rlBegin(RL_LINES);
+				// Three lines, six points
+				// Define color for next vertex
+				rlColor4ub(255, 0, 0, 255);
+				// Define vertex
+				rlVertex2f(trianglePositions[0].x, trianglePositions[0].y);
+				rlColor4ub(0, 255, 0, 255);
+				rlVertex2f(trianglePositions[1].x, trianglePositions[1].y);
+
+				rlColor4ub(0, 255, 0, 255);
+				rlVertex2f(trianglePositions[1].x, trianglePositions[1].y);
+				rlColor4ub(0, 0, 255, 255);
+				rlVertex2f(trianglePositions[2].x, trianglePositions[2].y);
+
+				rlColor4ub(0, 0, 255, 255);
+				rlVertex2f(trianglePositions[2].x, trianglePositions[2].y);
+				rlColor4ub(255, 0, 0, 255);
+				rlVertex2f(trianglePositions[0].x, trianglePositions[0].y);
+            rlEnd();
+        }
+        else
+        {
+            // Draw triangle as a triangle
+            rlBegin(RL_TRIANGLES);
+				// One triangle, three points
+				// Define color for next vertex
+				rlColor4ub(255, 0, 0, 255);
+				// Define vertex
+				rlVertex2f(trianglePositions[0].x, trianglePositions[0].y);
+				rlColor4ub(0, 255, 0, 255);
+				rlVertex2f(trianglePositions[1].x, trianglePositions[1].y);
+				rlColor4ub(0, 0, 255, 255);
+				rlVertex2f(trianglePositions[2].x, trianglePositions[2].y);
+            rlEnd();
+        }
+
+        // Render the vertex handles, reacting to mouse movement/input
+        for (unsigned int i = 0; i < 3; i++)
+        {
+            Vector2 position = trianglePositions[i];
+
+            float size = 4.0f;
+
+            Vector2 mousePosition = GetMousePosition();
+
+            // If the cursor is within the handle circle
+            if (Vector2Distance(mousePosition, position) < size)
+            {
+                float fillAlpha = 0.0f;
+                if (triangleIndex == -1)
+                {
+                    fillAlpha = 0.5f;
+                }
+
+                // If handle selected/clicked
+                if (i == triangleIndex)
+                {
+                    fillAlpha = 1.0f;
+                }
+
+                // If clicked, set selected index to handle index
+                if (IsMouseButtonPressed(MOUSE_BUTTON_LEFT))
+                {
+                    triangleIndex = i;
+                }
+
+                // If visible, draw DARKGRAY circle with varying alpha.
+                if (fillAlpha > 0.0f)
+                {
+                    Color fillColor = ColorAlpha(DARKGRAY, fillAlpha);
+
+                    DrawCircleV(position, size, fillColor);
+                }
+            }
+
+            // Draw handle outline
+            DrawCircleLinesV(position, size, BLACK);
+        }
+
+        // Draw controls
+        DrawText("space for lines\nleft for backface culling\nright for no backface culling\nclick and drag points\nr to reset", 10, 10, 20, DARKGRAY);
+
+        EndDrawing();
+        //----------------------------------------------------------------------------------
+    }
+
+    // De-Initialization
+    //--------------------------------------------------------------------------------------
+    CloseWindow();        // Close window and OpenGL context
+    //--------------------------------------------------------------------------------------
+
+    return 0;
+}