loader/dri3: Allocate up to 4 back buffers for page flips
Patch 3 is the main change, fixing low frame-rates with swap interval 0 via Xwayland on Wayland compositors which can directly scan out from client buffers.
Patch 1 prevents this from always allocating more buffers, and patch 2 makes sure the number of buffers doesn't oscillate when the X server skips some of the frames submitted to it.