Let the UI layer (via the core) construct the frame mask

This essentially just moves install_corners() from the compositor, through
the core, into the UI layer where it arguably should have been anyway,
leaving behind stub functions which call through the various layers. This
removes the compositor's special knowledge of how rounded corners work,
replacing it with "ask the UI for an alpha mask".

The computation of border widths and heights changes a bit, because the
width and height used in install_corners() are the
meta_window_get_outer_rect() (which includes the visible borders but not
the invisible ones), whereas the more readily-available rectangle is the
MetaFrame.rect (which includes both). Computing the same width and height
as meta_window_get_outer_rect() involves compensating for the invisible
borders, but the UI layer is the authority on those anyway, so it seems
clearer to have it do the calculations from scratch.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=697758


Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Jasper St. Pierre <jstpierre@mecheye.net>