xwayland: pass the emulated mode by reference

When using xrandr emulation, the emulated mode is passed as a pointer to
the XRandR mode from the xwl_output associated with the X11 client.

In preparation for fullscreen mode, we want to be able to reuse that
code but use a separate emulated mode.

Simply change the internal API to pass a reference to the emulated mode.

This introduces no functional change.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>