WM: Only update magnification bounds when they are visible

Fixes an issue where the magnification bounds would continuously
update whenever the bounds change, even when the magnification
overlay was not actually visible.

This caused a lot of jank when resizing the stacks in split screen.

Change-Id: Iad801b06c4cba6b972553e377692491f133be40e
Fixes: 78276993
Test: Enable magnification, enter split screen mode, drag resize handle, verify theres no jank