Add support for per client randr-resolution change emulation using viewport,
for apps which want to change the resolution when going fullscreen.

Partly based on earlier work on this by Robert Mader <robert.mader@posteo.de>

Note SDL2 and SFML do not restore randr resolution when going from
fullscreen -> windowed, I believe this is caused by us still reporting the
desktop resolution when they query the resolution.  This is not a problem
because when windowed the toplevel window size includes the window-decorations
so it never matches the emulated resolution.

One exception would be the window being resizable in Windowed mode and the
user resizing the window so that including decorations it matches the
emulated resolution *and* the window being at pos 0x0. But this is an
extreme corner case. Still I will submit patches upstream to SDL2
and SFML to always restore the desktop resolution under Xwayland,
disabling resolution emulation all together when going windowed.

Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>
Acked-by: Michel Dänzer <mdaenzer@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>