vcl: The backend should decide whether fast alpha drawing is available.

The unx drawAlphaBitmap() already already checks that the source and
destination have the same size.

Windows and OpenGL should be able to handle that without trouble.

OS X would need some additional code, so added an early return for the case
the source and result sizes do not match.

Change-Id: Ib09654aaa9c9064c232cff5094fcaa1190e931c6