As planned and shown in the mockups for GNOME 40, round the corners of
the background wallpaper of workspaces.
To do that we use the new rounded-clipping support of
MetaBackgroundContent and we animate the radius by attaching it to the
stateAdjustment just like everything else.
Because we show only a part of the wallpaper and "cut away" the area of
the panel in WorkspaceBackgrounds vfunc_allocate(), we also need to set
the rounded clips bounding rect to the rectangle we're actually showing,
otherwise the texture would be rounded in the region that's cut away.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1651>
63cf60b731