Replaced GLEW by GLAD

Removed GLEW external dependency, now it works with GLAD
Kept GLEW path, just in case... detected weird behaviour when testing
with gDEBugger

src/core.c

@@ -55,7 +55,7 @@
 
 #if defined(PLATFORM_DESKTOP) || defined(PLATFORM_WEB)
 
-    #define GLEW_EXTENSIONS_LOADER
+    #define GLAD_EXTENSIONS_LOADER
     #if defined(GLEW_EXTENSIONS_LOADER)
         #define GLEW_STATIC
         #include <GL/glew.h>        // GLEW extensions loading lib

src/rlgl.c

@@ -46,8 +46,9 @@
     #ifdef __APPLE__                // OpenGL include for OSX
         #include <OpenGL/gl3.h>
     #else
-        #include <GL/glew.h>        // GLEW header, includes OpenGL headers
-        //#include "glad.h"         // glad header, includes OpenGL headers
+        //#define GLEW_STATIC
+        //#include <GL/glew.h>        // GLEW header, includes OpenGL headers
+        #include "glad.h"         // glad header, includes OpenGL headers
     #endif
 #endif