XPending() will do a recvmsg() syscall if there are no items in the queue.
In most cases, this is unnecessary because we know that there is data to be
read of the connection or there are items already read which simply need
to be processed.
Discovering both of those conditions can be done without recvmsg() in the
hot paths.
Before this path, every iteration of the main loop had the potential to
submit a recvmsg() syscall. This reduces that overhead drastically.
XFlush() on the other-hand knows if it needs to write data or not and will
do no IO in the case the buffer is empty.
Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/3653
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4006>
Do the few remaining things that GDK is doing for us:
- Open and close the X11 Display
- Set up a GSource on the Display FD to handle events
- Allocate and free the content of XGenericEventCookie,
to "unroll" the few XInput2 events that Mutter still
does handle.
And remove the GdkDisplay we've so long relied on.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2864>