Steps to repro:

1. Open attached project and 'test' scene on OSX;
2. Select Terrain gameobject in the Hierarchy;
3. Select 'Raise/Lower Terrain' tab in the Inspector;
4. Select any brush (preferably the one that is more complicated, e.g. one of the last three);
5. Hover on terrain, click and hold left mouse button and paint height on terrain.

Actual result:
Visual artifacts appear on terrain in Scene View (see attached video).
Not a regression, since it happens only newly introduced Metal platform.

Expected result:
Artifacts should not appear on terrain when modifying its heightmap.

Notes:
- Reproducible only on OSX 10.12.1;
- Reproducible in 5.6.0a5;
- Not reproducible on WIndows 10;
- Reproducible only in Scene view/Not reproducible in Game view;
- Not reproducible in GlCore graphics API.