Surfaces that don't cover the whole output aren't centered with xdg_toplevel::set_fullscreen #8981
Description
Reproduction steps
Run weston-simple-egl -f -r
Observation
The surface starts at 0,0 of the output instead of being centered and pillar/letter boxed as implied by the xdg-shell specification:
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).