wayland: fix vertical resizing

Resizing in strictly the vertical direction has been broken forever on
wayland. It's because of how the keepaspect calculation always resized
with respect to the width. So if you moved the mouse strictly
vertically with no change left/right, the mpv window would never change
size since it just calculates with respect to the width that doesn't
change.

Fixing this is kind of weird and maybe someone should think of a better
algorithm for preserving aspect ratio (we just multiply by the gcd
basically), but whatever. You might naively try something like "resize
with respect to what direction changes the most" at first, but this
leads to very cludgy resizing since during a typical mouse dragging
motion, it will flip constantly from "resize w/ respect to the width"
and "resize w/ respect to the height". It "works" but it's really ugly
and not worth it.

The trick is to realize that we need to consider the resizing state as
one continuous motion, choose a direction initially, stick to it
throughout the entirety of the resize, and reset the relevant parameters
after we finish the resize. This ensures the direction never
unintuitively flips during a motion, but also allows for the strictly
vertical case to still work.

Might as well note it here in the commit, but mpv's resizing behavior
technically violates xdg-shell. For the resizing event, "[t]he window
geometry specified in the configure event is a maximum; the client
cannot resize beyond it." Well, we do obviously go beyond the maximum in
certain cases. For example consider resizing strictly horizontally. The
width value increases from the compositor but not the height. Since mpv
preserves aspect ratio by default, the height obviously must increase
which will go over the prescribed bounds by the compositor. This happens
before and after this commit, and I think everyone agrees is the desired
behavior. With this commit, now vertical resizing works which violates
xdg-shell in the same way. Before, it incidentally obeyed the protocol
since it did not resize at all, but let's presume users expect the
window to actually get bigger when they do a drag resize.

*: https://wayland.app/protocols/xdg-shell#xdg_toplevel:enum:state:entry:resizing