workspaceThumbnail: Use current number of workspaces for sizes

workspaceThumbnail: Use current number of workspaces for sizes

There are situations where MetaWorkspaceManager and ThumbnailsBox disagree
on the number of workspaces, for example when animating them out. It's more
important to follow the visible number of workspaces while they're updated.

Make vfunc_get_preferred_width() and vfunc_get_preferred_height() use the
current number of workspace thumbnails to calculate their sizes, instead of
MetaWorkspaceManager's n-thumbnails property.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1593>