Improved graphics engine and sprite handling

[lschmid83]

The graphics engine for the game was built based on a very old Android sprite method test demo. I propose a complete rewrite based on modern Open GL ES techniques. This would improve the overall framerate and mean levels could be larger with more sprites displayed on the screen at one time. A potential improvement could be made to the drawing of textures by instead of drawing each tile of the same time separately in each frame all tiles of the same type are drawn in one routine.