GreGetDIBitsInternal(): fix INT/UINT return type mismatch

(Noticed while looking at CORE-13129.)

GreGetDIBitsInternal()

 672 INT

 673 APIENTRY

 674 GreGetDIBitsInternal(

 675     HDC hDC,

 676     HBITMAP hBitmap,

 677     UINT StartScan,

 678     UINT ScanLines,

 699         return 0;

1049     return ScanLines;

Idea:
Add an explicit cast and probably something like ASSERT(ScanLines <= MAX_INT).
Unless this function should actually return an UINT.

NB:
I wonder why builders do not report this "warning"?