WARNING: BIG rewrite of rlgl module

This commit implements a big update of rlgl module, intended to optimize some parts. This change could break some code bases... hopefully not, but it could.
The BIG changes to the module are:
 - Replaced LINES-TRIANGLES-QUADS buffers by a single one, now all vertex data is accumulated on a single buffer and managed with registered draw calls. LINES-TRIANGLES-QUADS could be used the same way as before, rlgl will manage them carefully. That's a big improvement of the system.
 - Support multi-buffering if required. Just define MAX_BATCH_BUFFERING desired size (currently set to 1 batch). Should be enough for most of the situations.
 - Removed temporal accumulative buffers for matrix transformations, now transformations are directly applied to vertex when on rlVertex3f()
 - Reviewed rlPushMatrix()/rlPopMatrix() to be consistent with OpenGL 1.1, probably I should remove that ancient behaviour but... well, it was not consistent and now it is.
 - Minor tweaks: LoadText(), I broke it in last update... also multiple comments reviewed.
 - TODO: MAX_BATCH_ELEMENTS checking should probably be reviewed... done some tests and it works but...

examples/models/models_billboard.c

@@ -28,7 +28,6 @@ int main()
     camera.fovy = 45.0f;
     camera.type = CAMERA_PERSPECTIVE;
     
-
     Texture2D bill = LoadTexture("resources/billboard.png");     // Our texture billboard
     Vector3 billPosition = { 0.0f, 2.0f, 0.0f };                 // Position where draw billboard
     
@@ -52,11 +51,11 @@ int main()
             ClearBackground(RAYWHITE);
 
             BeginMode3D(camera);
-            
-                DrawBillboard(camera, bill, billPosition, 2.0f, WHITE);
                 
                 DrawGrid(10, 1.0f);        // Draw a grid
-
+                
+                DrawBillboard(camera, bill, billPosition, 2.0f, WHITE);
+                
             EndMode3D();
 
             DrawFPS(10, 10);