0x00 EXE fence. Signals when command stream interpreter has reached the point
where the fence was emitted.
0x01 FLUSH fence. Signals when command stream interpreter has reached the point
where the fence was emitted, and all previous drawing operations have been
completed and flushed.
Implements busy wait (for fastest response time / high CPU) and
lazy wait (User interrupt or timer driven).
Thomas Hellstrom