Fix GDI return values and other fixes

Fix GDI return values and other fixes

* 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