to the context struct to hold origin of drawable in draw buffer. The
viewport is adjusted using these fields.
Enabled scissor test, drm now uses scissor values from the sarea context.
Also enabled color masking, drm uses mask from sarea context
(untested).
The viewport calcs seem to be working, but cliprects still have problems
for single buffer contexts and overlapping GL windows. Needs more
testing and debugging. I'm committing now so we can continue with
testing.