fdo#74124: Scale the pictures before calling ImplDrawAlpha().

When the source and destination bitmap do not have the same size,
ImplDrawAlpha() does not use direct paint, but instead it gets the image from
the screen, blends it with the provided bitmap, and again draws it.

Unfortunately, the blending uses the most trivial (and ugly) way of scaling;
so to produce much better results, let's scale to the destination size before
even calling ImplDrawAlpha().

The sideeffect is that we use the direct paint in most cases now; so hopefully
it pays off to do the (a bit more expensive) scale first.

Change-Id: I3b6b275710220910709ae4345ad6be3d6e4bf79c