wayland: handle multiple outputs more correctly

There's a bit of a catch-22 in the wayland backend. mpv needs to know
several things about the wl_output the surface is on (geometry, scale,
etc.) for lots of its options. You still have to render something
somewhere before you can know what wl_output the surface is actually on.
So this means that when initializing the player, it is entirely possible
to calculate initial parameters using the wrong wl_output. The surface
listener is what will eventually correct this and pick the correct
output. However not everything was technically working correctly in a
multi-output setup.

The first rule here is to rework find_output so that it returns a
vo_wayland_output instead of internally setting wl->current_output. The
reason is simply because the output found here is not guaranteed to be
the output the surface is actually on. Note that for initialization of
the player, we must set the output returned from this function as the
wl->current_output even if it is not technically correct. The surface
listener will fix it later.

vo_wayland_reconfig has to confusingly serve two roles. It must ensure
some wayland-related things are configured as well as setup things for
mpv's vo. The various functions are shuffled around and some things are
removed here which has subtle implications. For instance, there's no
reason to always set the buffer scale. It only needs to be done once
(when the wl->current_output is being created). A roundtrip needs to be
done once after a wl_surface_commit to ensure there are no configuration
errors.

surface_handle_enter is now handles two different things: scaling as
well as mpv's autofit/geometry options. When a surface enters a new
output, the new scaling value is applied to all of the geometry-related
structs (previously, this wasn't done). This ensures, in a multi-monitor
case with mixed scale values, the surface is rescaled correctly to the
actual output it is on if the initial selection of wl->current_output is
incorrect.

Additionally, autofit/geometry values are recalculated if they exist.
This means that dragging a surface across different outputs will autofit
correctly to the new output and not always be "stuck" on the old one.

A very astute observer may notice that set_buffer_scale isn't set when
the surface enters a new output. The API doesn't really indicate this,
but a WAYLAND_DEBUG log reveals that the compositor (well at least
sway/wlroots anyway) magically sets this for you. That's quite fortunate
because setting in the surface handler caused all sorts of problems.

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