Repro Steps:
1. Create a Terrain
2. Create a Terrain Layer and assign a Texture
3. Select the created Terrain
4. Select the Paint Texture Tool
5. Add the Terrain Layer to the Terrain via the Paint Texture Tool UI
6. Open Project Settings and go to Quality Settings
7. Set texture Quality to something that is NOT Full Res
8. Try painting texture on Terrain
9. Press the Play Button to enter Play mode

Result: Cannot paint on Terrain with non-FullRes texture quality. The Editor UI freezes up (as if Repaint doesnt happen) though there is still some functionality available like right-click context menus, maybe selecting a EditorWindow tab, and entering Play Mode

Expectations: Can still paint on Terrain with any Texture Quality. UI doesnt freeze up. Editor doesnt crash