Mutter does not enforce that buffer size is a integer multiple of buffer_scale (retry again)

The Wayland core protocol states:

The new size of the surface is calculated based on the buffer
size transformed by the inverse buffer_transform and the
inverse buffer_scale. This means that at commit time the supplied
buffer size must be an integer multiple of the buffer_scale. If
that's not the case, an invalid_size error is sent.

This was reported initially in #1938 (closed) and fixed with !2188 (merged).

Unfortunately, that introduced a regression with cursors are these may not be divisible by the scale (as discussed in https://gitlab.freedesktop.org/wayland/wayland/-/issues/194) so the change was reverted in !2335 (merged) to fix #2083 (closed).

So mutter still does not enforce the size requirement and applications may violate that rule without noticing when running on GNOME Shell (e.g. https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/733#note_1738935).

This is a request to re-instantiate the buffer size requirement in mutter, excluding the cursors from the rule.

/cc @emersion @nielsdg @rmader