Renamed shader variables (more generic names)

Now shader maps use a generic naming convention for any kind of texture
maps (not only diffuse, normal or specular). Useful for custom shaders.

src/rlgl.h

@@ -171,10 +171,10 @@ typedef enum { OPENGL_11 = 1, OPENGL_33, OPENGL_ES_20 } GlVersion;
         int mvpLoc;           // ModelView-Projection matrix uniform location point (vertex shader)
         int tintColorLoc;     // Color uniform location point (fragment shader)
         
-        // Texture map locations
-        int mapDiffuseLoc;    // Diffuse map texture uniform location point (fragment shader)
-        int mapNormalLoc;     // Normal map texture uniform location point (fragment shader)
-        int mapSpecularLoc;   // Specular map texture uniform location point (fragment shader)
+        // Texture map locations (generic for any kind of map)
+        int mapTexture0Loc;  // Map texture uniform location point (default-texture-unit = 0)
+        int mapTexture1Loc;  // Map texture uniform location point (default-texture-unit = 1)
+        int mapTexture2Loc;  // Map texture uniform location point (default-texture-unit = 2)
     } Shader;
 
     // Texture2D type