Only happens with negative height stamp values. Might not be an issue on Chris's branch but making a bug report just to keep track of this

1. Create Terrain
2. Select Create Neighbor Terrain tool
3. Create neighboring Terrains
3. Select Stamp Height Tool
4. Shift + scroll mouse wheel up to get a positive stamp height
5. Stamp Terrain and neighboring Terrains with positive stamp height
6. Shift + scroll mouse wheel down to get a negative stamp height
7. On the selected Terrain Tile, stamp negative height
8. Stamp preview should still show that the stamp height is negative
9. Stamp on a neighboring Terrain
10. Negative stamp height is clamped to 0
11. User has to scroll to a negative stamp height again