workspace: Unbreak reposition animation

Commit 1ea22a52 broke the window reposition animation when it based the ::size-changed signal on the layout manager's bounding box instead of the MetaWindow::size-changed signal.

That happening because of the combination of:

  1. we adjust to window size changes immediately without animations
  2. closing a window triggers a change to a 0x0 bounding box which is not treated as a size change

Fix this by addressing the 2nd factor, and don't treat a change to a 0x0 bounding box as size change.

Fixes #2901 (closed)

Merge request reports