xwayland: Poll dma-buf fds in xwl_present_pixmap
Wait for all of them to become readable before processing the
PresentPixmap
request. This corresponds to the GPU drawing to the buffer
having completed.
This has various benefits, e.g. the target MSC calculation can be more accurate in some cases.
A potential downside is that it may impact the frame rate of very light
clients such as vblank_mode=0 glxgears
. Shouldn't affect real-world apps
though.