This pull request prevents shadows being rendered for left and right side titled windows. This behaviour is consistent with maximised windows, which also do not render shadows.

The rationale for this change is so that when two windows are titled along side each other, it prevents central shadows bleeding into the touching points of the windows.

metacity-theme-x.xml has provision to style left/right titled windows. This patch makes it possible to to create window themes that present clean side-by-side tiled windows.

An ARGB window with a frame is likely something like a transparent
terminal. It looks awful (and breaks transparency) to draw a big
opaque black shadow under the window, so clip out the region under
the terminal from the shadow we draw.

Add meta_window_get_frame_bounds() to get a cairo region for the
outer bounds of the frame of a window, and modify the frame handling
code to notice changes to the frame shape and discard a cached
region. meta_frames_apply_shapes() is refactored so we can extract
meta_frames_get_frame_bounds() from it.

https://bugzilla.gnome.org/show_bug.cgi?id=635268

NOTE: Applied only partially, compositor part is still missing...

upstream commit:
https://gitlab.gnome.org/GNOME/metacity/commit/0f2e32d1

NOTE: Patch copied from mutter and adapted for metacity.

based on metacity commit: 72d1c6a6b9a85391444bb5ff8c5de1e3aabd3fd9
From: Alberts Muktupāvels <alberts.muktupavels@gmail.com>