The new Qt 4.5 raster graphics system has *horrible* performance with this screensaver, so try to do the painting in a more uniformly performant manner.

This seems to make the X11 engine very slightly slower but it makes the raster engine *much* faster (and OpenGL still works...).

Will backport due to the extreme amount of slowdown experienced.

Major changes are to use Source composition mode in a lot of spots to avoid using alpha-blended bitBlts.
In addition the back buffer is always full blitted to the window, the dirty regions are ignored now.