loader/dri3: Do not eagerly drop back buffers on copy presents
The X server can randomly decide to use COPY for a present even if the client has not changed the swap interval. If we drop the back buffers at that point, we lose their contents, which clients might not expect.
This also has performance implications, since eagerly dropping everything might cause us to thrash buffer frees/allocs.
Fixes KWin random black/garbage flickering on asahi/X11.
Fixes: 60585fc4 ("loader/dri3: Only allocate additional buffers if needed") Signed-off-by: Asahi Lina lina@asahilina.net