Bug 461063

SUMMARY

From the xdg-shell spec:
```
If the surface doesn't cover the whole output, the compositor will
position the surface in the center of the output and compensate with
with border fill covering the rest of the output. The content of the
border fill is undefined, but should be assumed to be in some way that
attempts to blend into the surrounding area (e.g. solid black).

If the fullscreened surface is not opaque, the compositor must make
sure that other screen content not part of the same surface tree (made
up of subsurfaces, popups or similarly coupled surfaces) are not
visible below the fullscreened surface.
```

STEPS TO REPRODUCE
1. run `weston-simple-egl -f -r` (requires very recent weston version, not released yet)

OBSERVED RESULT
The client surface is not centered, content behind it is visible.

EXPECTED RESULT
The client surface is not centered, content behind it is covered.

ADDITIONAL INFORMATION
This is currently not widely used by clients, however Wine-Wayland, once merged and enabled, will rely on it. Similar bugs are or where present in Mutter and Exo.