vcl: try to fix sizes in ImplBlendToBitmap()

Since commit 2f84b420,
sw_unoapi test sw.SwAccessibleEndnoteView crashes while trying to blend
a 1107x8 Bitmap into a 97x8 Bitmap; it's not obvious that any of the
parameters passed in is wrong and it's odd that ImplBlendToBitmap()
would use rSrcBuffer.mnHeight and rDstBuffer.mnWidth; presumably
it should use both height and width of the same buffer.

Change-Id: I40b98ba56e703323df91f6fd7773addcfd69a402

vcl/source/gdi/bmpfast.cxx

@@ -608,7 +608,8 @@ bool ImplBlendToBitmap( TrueColorPixelPtr<SRCFMT>& rSrcLine,
         nDstLinestep = -nDstLinestep;
     }
 
-    for( int y = rSrcBuffer.mnHeight; --y >= 0; )
+    assert(rDstBuffer.mnHeight <= rSrcBuffer.mnHeight); // not sure about that?
+    for (int y = rDstBuffer.mnHeight; --y >= 0;)
     {
         ImplBlendLines<8>( aDstLine, rSrcLine, aMskLine, rDstBuffer.mnWidth );
         aDstLine.AddByteOffset( nDstLinestep );