xdg-shell: Soften fullscreen geometry requirements

Having a strict requirement on clients obeying the configured window
geometry for fullscreen toplevel surfaces might have the side effect of
making it harder or impossible to implement various hardware
optimizations on certain system configurations. By softening
requirements on the geometry while loosely defining the border fill, we
remove that restriction.

Clients that still want total control of the surrounding area can
still for example prepare the attached buffers to match the configured
surface size, or use subsurfaces in combination with wp_viewporter to
make up a surface matching the fullscreen window geometry dimensions.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Arnaud Vrac <rawoul@gmail.com>
Reviewed-by: Mike Blumenkrantz <zmike@samsung.com>