workspace: Avoid redundant boundingBox copies in window layout

workspace: Avoid redundant boundingBox copies in window layout

WorkspaceLayout's allocation reads child.boundingBox four times for each
window on every frame of an overview transition. The getter returns a
fresh copy of the cached box on every access, so this is four throwaway
objects per window per frame feeding the garbage collector.

Read the box into a local once and use that. The values are identical;
only the per-frame allocations go away.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4250>
(cherry picked from commit d4b5bc781535104475fba490fccfb0dd7934c96f)