wayland: only perform a rescale if window is on one output

The mpv window overlapping multiple outputs with different scale values
can result in some weird behavior when dragging it from one monitor to
another one. This is due to the way some compositors implement
preferred_scale or preferred_buffer_scale (integer scale equivalent).
Depending on the scale values, mpv window has to be resized to match the
new scaling value (due to fractional scaling requiring a viewport). This
can cause the window to become smaller and no longer overlap the monitor
you were just trying to drag it to. Repeat this and the window will
become smaller and smaller. Depending on the layout, the reverse can
also happen (the window becomes larger). This can cause additional
events to fire as the preferred_scale value may change again which does
more weird things.

It seems kwin is not affected by this because their implementation of
preferred_scale sends the event only if the window is fully on the new
monitor. Honestly, this is probably more logical anyway but we should at
least deal with the other implementations better. Try to deal with it by
reworking scaling changes so they only occur when the mpv window is
fully on one monitor. If we get a preferred_scale event and there is an
overlap, save it as a pending change to be performed on the next
surface_enter or surface_leave event (whichever results in there being
only one monitor. Some weird rendering glitches can still happen during
overlap but this makes it usable again.

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