[WIN32SS:GDI] Fix an off-by-one error in the StretchBlt mask bounds check (#9136)

The mask bounds check in `DIB_XXBPP_StretchBlt` allowed `sx == cx` (and similar for `sy`),
reading one pixel past the end of the mask bitmap.
On the contrary, the source-surface check a few lines below already does it right.

win32ss/gdi/dib/stretchblt.c

@@ -174,7 +174,7 @@ BOOLEAN DIB_XXBPP_StretchBlt(SURFOBJ *DestSurf, SURFOBJ *SourceSurf, SURFOBJ *Ma
           sx = SourceRect->left+(DesX - DestRect->left) * SrcWidth / DstWidth;
         }
         if (sx < 0 || sy < 0 ||
-          MaskSurf->sizlBitmap.cx < sx || MaskCy < sy ||
+          MaskSurf->sizlBitmap.cx <= sx || MaskCy <= sy ||
           fnMask_GetPixel(MaskSurf, sx, sy) != 0)
         {
           CanDraw = FALSE;