[Core] Simplified and cleaned up Renderer interfaces;
[Artwork] Added WIP DefaultLayout file for PolyCode UITheme;
[Core] Fixed bug regarding not properly rendering all the time;
[Core] Added Lua Globals Tracker utility class;
[Core] Fixed a bug on UI Custom Properties regarding not properly interpreting the Default Value;
[Core] Added Clipping support to the ClipRect Stack on Renderer to prevent overflowing ClipRects;
[Core] Removed GLCHECK();