Fix calculation of destination pivot point in GPU_BlitRectX
Fix calculation of destination pivot point in GPU_BlitRectX

Fixes #46

ademuri/sdl-gpu,grimfang4/sdl-gpu,grimfang4/sdl-gpu,grimfang4/sdl-gpu,grimfang4/sdl-gpu,ademuri/sdl-gpu,ademuri/sdl-gpu,ademuri/sdl-gpu