xdg-shell: Specify fullscreen size-mismatch handling
xdg-shell: Specify fullscreen size-mismatch handling

We cannot rely on the client to provide a surface filling the output so
we must specify what happens with the outputs area that is not covered
completely.

Signed-off-by: Jonas Ådahl <67dcd8a17e758192e0cbb577ce1d62cc7da6cf49@gmail.com>
Reviewed-by: Bryce Harrington <bdb14cfc1ab8d24e13de84adc40b41ae60c13bd1@osg.samsung.com>
Reviewed-by: Pekka Paalanen <0f6ae7001e34c37b02fb2c1c2573ddf8f442136a@collabora.co.uk>

kwm81/weston,xorgy/weston,giucam/weston,eyolfson/weston,sir-murray/weston,Tarnyko/weston-xdg_surface_present,jonnylamb/weston,jonnylamb/weston,Fantu/compositor-spice,krezovic/weston,Fantu/compositor-spice,eyolfson/weston,Gnurou/weston,mchalupa/weston,mchalupa/weston,udoprog/weston,udoprog/weston,kwm81/weston,xorgy/weston,Gnurou/weston,sir-murray/weston,giucam/weston,Fantu/compositor-spice,krezovic/weston,Tarnyko/weston-xdg_surface_present