solid-fill-based composite acceleration.
- Use a real pixmap when doing an UploadToScratch (For pDrawable->type ==
DRAWABLE_WINDOW, you need to get the backing pixmap).
- Pass back the x/y offsets from kaaGetOffscreenPixmap unconditionally,
because they'll be used in the scratch case.
- Turn on the Render acceleration for Rage 128 and Radeon 100-series at
last!