Reworked software screen compositing. Uses graphics library region primitives instead of duplicating the work to handle display compositing. use a backfill hook to render empty display regions.
# Compositing on all sides of screens works but is set to amigaos like behaviour by default (composites above, fills left/right/below)
# Rendering artefacts have been fixed, aswell as issues with screen y coord being reset.
# bug preventing dragging of screens below the visible bounds (investigating)
git-svn-id: https://svn.aros.org/svn/aros/trunk/AROS@48126 fb15a70f-31f2-0310-bbcc-cdcc74a49acc