workspace: Extend clip when animating from window picker to app grid

workspace: Extend clip when animating from window picker to app grid

We want to clip the Workspace actor in the appGrid state of the overview
in order to make sure windows that overflow the monitor don't spill out.
So far we had commit b1970b95b8813c00895f3da9f4fc88ca04627954 for that.

Now since the last commit, window previews always slightly overflow the
allocation with their icons. That means a part of the window icon gets
clipped away as soon as the transition to the appGrid starts, which
looks weird.

Fix that bug in the transition by slightly extending the clip downwards
when animating between the window picker and the app grid state. The
extra height we extend the clip by is controlled by the overviewState,
which means we extend the clip by the full icon overlap in the window
picker state, but don't extend the clip at all when in the app grid
state.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1813>