windowManager: Fix switch animation between non-adjacent workspaces

windowManager: Fix switch animation between non-adjacent workspaces

Commit e5c95b910d refactored the workspace animation to also handle
animations that involve all surrounding workspaces, but due to an
ill-advised review comment (guess whose) it broke the animation
for non-neighboring workspaces.

Update the code to handle correctly whether in a given direction:
 - we have the target workspace of a given index
 - we have a neighboring workspace
 - we don't need to animate anything

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/182