window: Move get_toplevel_xwindow to window-x11
window: Fix get_input_rect in a hacky way

All of the users of get_input_rect don't actually want a synthesized
input rect based off of the current margins. What they really want is
the last-configured size of the toplevel window.

Since we don't properly track this anymore in the generic MetaWindow,
use XGetWindowAttributes to fetch a server-side rectangle. This is a
bad layer violation, but since the window geometry code will have to
be rewritten anyway for the Wayland set_window_geometry, let's just
push a hacky fix for now.