Rework backbuffer flushing
This MR has 3 commits:
- the first one is radeonsi specific to avoid unnecessary work
- the second one is adding a callback to st->flush that is called after the states have been flushed but before the flush to the hardware. This is allowing users of st->flush to push some work that will be executed last.
- the last one use the callback to rework backbuffer flushing