using glad for opengl
removed simple test of glad
added missing include/ files from glad
No more immediate mode! VBOs! New Texture!

Texture no longer depends on SDL's Texture capabilities, and supports 24-
and 32-bit color.

TileLayers now use OpenGL Vertex Buffer Objects (VBOs) to upload its data
less frequently and more efficiently. (Right now, use the new
refreshTiles() method to do the upload.)