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.