Added depth drawing shader

NOTE: It requires a depth texture as input, it should be configured on
rlgl, by default RenderTexture (fbo) uses Depth Renderbuffer instead of
Depth Texture. Check rlglLoadRenderTexture()

examples/resources/shaders/glsl330/depth.fs

@@ -0,0 +1,27 @@
+#version 330
+
+// Input vertex attributes (from vertex shader)
+in vec2 fragTexCoord;
+in vec4 fragColor;
+
+// Input uniform values
+uniform sampler2D texture0;     // Depth texture
+uniform vec4 fragTintColor;
+
+// Output fragment color
+out vec4 finalColor;
+
+// NOTE: Add here your custom variables
+
+void main()
+{
+    float zNear = 0.01; // camera z near
+    float zFar = 10.0;  // camera z far
+    float z = texture(texture0, fragTexCoord).x;
+
+    // Linearize depth value
+    float depth = (2.0*zNear)/(zFar + zNear - z*(zFar - zNear));
+    
+    // Calculate final fragment color
+    finalColor = vec4(depth, depth, depth, 1.0f);
+}