the flagged bits from a source picture. Approved in principle by
keithp.
- Use CopyPicture and SetTransform to update most of the backing picture's
state in the composite wrapper. Filters are still missing.
- Don't allocate a picture private, now that we calculate clipping properly
and don't need the serialNumber or stateChanges.
- Use the format of the source pixmap rather than generating the format
from the window's visual.
- Wrap the rest of the Render primitives that were stubbed out before.
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