Fix sprite drawing

Sprite now draws correctly - the main culprit was a wrong comparison when checking for the end of the sprite.
Movement is also fixed - at least in Y direction. The sprite now moves between the top and bottom edge of the screen - no wrapping is done. X movement is commented out for the time being.

Weird things happened when moving the sprite because signed comparisons ( checked using BPL and BMI) were used, but the numbers being worked with were unsigned - more on this here: http://www.6502.org/tutorials/compare_instructions.html