Add Remove Squashed Layers

Test that layers can be nicely added or removed from a squashed layer, without unnecessary repaints on any layer. Click anywhere to test interactively; keep clicking to proceed through the test.

CASE 1, original layer tree with overlap1 and overlap2:

CASE 2, overlap3 gets added:

CASE 3, overlap2 gets removed. Since this does not resize the layer, there should only be a repaint of overlap2:

CASE 4, overlap1 gets removed:

CASE 5, overlap2 gets added back:

CASE 6, overlap1 gets added back, and overlap3 gets removed: