See if tags can be easily replaced.

Added rectangle drawing functions which accept a GPU_Rect: GPU_Rectan…

…gle2(), GPU_RectangleFilled2(), GPU_RectangleRound2(), and GPU_RectangleRoundFilled2().

Reorganized project layout. Moved includes into include/ and source f…

…iles into src/. Renamed some source files. Updated CMakeLists and Android.mk.