For example, when a theme part has a source image with 0xff00ff as transparent pixels and the sizing
margin is larger than the destination, the source image is first resized to fit the destination,
during which, StretchBlt() should be used instead of TransparentBlt() because the image should not
be blended with the temporary memory device context.

Fix group box background may be drawn in black.
Signed-off-by: Zhiyi Zhang <zzhang@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>