window: Rename get_input_rect to get_buffer_rect

With get_input_region existing, get_input_rect is a misnomer. Really,
it's about the geometry of the output surface, and it's only used that
way in the compositor code.

Way back when in GNOME 3.2, get_input_rect was added when we added
invisible borders. get_outer_rect was always synonymous with server-side
geometry of the toplevel. get_outer_rect was used for both user-side
policy (the "frame rect") and to get the geometry of the window.

Invisible borders were meant to extend the input region of the frame
window silently. Since most users of get_outer_rect cared about the
frame rect, we kept that the same and added a new method, get_input_rect
to get the full rect of the framed window with all invisible borders for
input kept on.

As time went on and CSD and Wayland became a reality, the relationship
between the server-side geometry and the "frame rect" became more
complicated, as can be evidenced by the recent commits. Since clients
don't tend to be framed anymore, they set their own input region.

get_buffer_rect is also sort of a poor name, since X11 doesn't really
have buffers, but we don't really have many other alternatives.

This doesn't change any of the code, nor the meaning. It will always
refer to the rectangle where the toplevel should be placed.
window: Save the buffer_rect internally

Rather than calculate it speculatively with the current properties
which may be too new or too out of date, make sure it always fits
with the proper definition. We update it when we update the toplevel
window for X11, and when a Wayland surface is committed with a newly
attached buffer.