xwayland/output: Set the "RANDR Emulation" property

Xwayland does not change the actual XRANDR setup for real, it just
emulates the resolution changes using viewports in Wayland.

With a single output, if an X11 applications tries to change the CRTC
back to the native mode, RRCrtcSet() will simply ignore the request as
no actual change is induced by this.

Set the property "RANDR Emulation" on all Xwayland outputs to make sure
the optimizations in RRCrtcSet() get skipped and Xwayland can receive
and act upon the client request.

Also make sure we do not allow that property to be changed by X11

v2: Prevent X11 clients from changing the property value
    (Pekka Paalanen <pekka.paalanen@collabora.com>)

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Closes: xorg/xserver#1305
(cherry picked from commit 7b7170ec)
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