workspace: Round the corners of the wallpaper

workspace: Round the corners of the wallpaper

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>