For Raise or Lower Terrain, only the last click will undo. The rest won't.

Repro steps:
1. Open SampleScene
2. Select the terrain
3. Choose the paint terrain tab
4. Choose "Raise or Lower Terrain" or any other tool
5. Choose NewBrush 2 (Note that this custom brush texture is set to "none")
6. Open the custom brush menu (lower left side of custom brush)
6. Click on the terrain 5 times
7. Press undo 5 times

Expected result: all 5 terrain clicks should undo
Actual result: Only the first click will undo, unless undo button is mashed enough