It always takes one update cycle for the copy to become visible on the
host windowing system, so waiting for the target MSC resulted in 1 cycle
delay.

We re-use the idle list for copies which were executed but need their
completion event sent.

Fixes black seams when resizing the "Builder" sub-window of

 GDK_BACKEND=x11 gtk4-demo

on Xwayland (see
https://gitlab.gnome.org/GNOME/mutter/-/issues/1290#note_873557

).

Unfortunately, this cannot completely fix the seams with apps which
queue up multiple frames in advance, since there's always at least one
queued frame corresponding to the old window size. But it should at
least help a little in that case as well.

v2:
* Bug fix: Don't update exec_msc in present_wnmd_check_flip_window.
  (Roman Gilg)
* Use exec_msc = target_msc - 1 instead of exec_msc--, and add a
  comment, for clarity.
v3:
* Drop exec_msc = target_msc again in present_wnmd_execute.
* present_execute_copy should never set vblank->queued in
  present_wnmd_execute now, so replace that branch with an assertion.
  (Roman Gilg)

Reviewed-by: Roman Gilg <subdiff@gmail.com>
Tested-by: Roman Gilg <subdiff@gmail.com>