desktop-shell: Allow fullscreen windows to mode-switch their output.

Fix desktop-shell's activate() method to only restore the output
mode on the single output on which a shell surface gets activated.

This way toplevel fullscreen surfaces can mode-switch their output
via method WL_SHELL_SURFACE_FULLSCREEN_METHOD_DRIVER and that
temporary mode properly persists until the surface loses its
fullscreen status, but effects like window switching and exposay
still work in the expected way.

v2: Split into a separate patch from original patch
    "Allow restore_output_mode() to work properly.",
    as suggested by Derek Foreman.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
1 file changed