Fixed GPU_GetVirtualCoords() so it inverts the coordinates when GPU_SetCoordinateMode() is used.
Fixed GPU_GetVirtualCoords() so it inverts the coordinates when GPU_SetCoordinateMode() is used.

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