workspaceAnimation: Block workspace updates during switch

workspaceAnimation: Block workspace updates during switch

WorkspaceAnimation uses a fixed set of workspace indices to transition
between for each gesture/animation.

That means that if workspaces change during the animation, the mapping
from workspace adjustment value to the animation progress can involve
invalid indices and the animation ends on the wrong workspace.

To fix this, block workspace updates during the animation.

Closes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/8148

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4014>
(cherry picked from commit eaa4e0577a08c3ed38b5f512b4af0b88ede7f1f5)