This is a workaround for X11 games which use randr to change the resolution
in combination with NET_WM_STATE_FULLSCREEN when going fullscreen.

Newer versions of Xwayland support the randr part of this by supporting randr
resolution change emulation in combination with using WPviewport to scale the
app's window (at the emulated resolution) to fill the entire monitor.

Apps using randr in combination with NET_WM_STATE_FULLSCREEN expect the
fullscreen window to have the size of the emulated randr resolution since
when running on regular Xorg the resolution will actually be changed and
after that going fullscreen through NET_WM_STATE_FULLSCREEN will size
the window to be equal to the new resolution.

We need to emulate this behavior for these games to work correctly.

Xwayland's emulated resolution is a per X11 client setting and Xwayland
will set a special _XWAYLAND_RANDR_EMU_MONITOR_RECTS property on the
toplevel windows of a client (and only those of that client), which has
changed the (emulated) resolution through a randr call.

This commit checks for that property and if it is set adjusts the fullscreen
monitor rect for this window to match the emulated resolution.

Here is a step-by-step of such an app going fullscreen:
1. App changes monitor resolution with randr.
2. Xwayland sets the _XWAYLAND_RANDR_EMU_MONITOR_RECTS property on all the
   apps current and future windows. This property contains the origin of the
   monitor for which the emulated resolution is set and the emulated
   resolution.
3. App sets _NET_WM_FULLSCREEN.
4. We check the property and adjust the app's fullscreen size to match
   the emulated resolution.
5. Xwayland sees a Window at monitor origin fully covering the emulated
   monitor resolution. Xwayland sets a viewport making the emulated
   resolution sized window cover the full actual monitor resolution.

!739