The coordinates from RDP ROP commands do not (always) have
0,0 as the origin of the drawing operation.
Adjust the coordinates to our local coordinate system.
* top level GDI functions return 0 on error and != 0 otherwise but the
low level functions (16bpp.c, 8bpp.c 32bpp.c) which are called did it
exactly the other way around. Those were adapted.
* change gdi_InvalidateRegion to BOOL and check calls where appropriate
* integrate comments from pull request
