Steps to repro:

1. Open attached project and t1 scene;
2. Select terrain gameobject;
3. Hover over terrain;
4. Observe terrain brush rendering.

Expected result:
Terrain brush is rendered smoothly without artifacts.

Actual result:
Artifacts appear on terrain brush. See attached video.

Notes:
- Reproducible in 2017.1.0b5, 5.6.0p4, 5.5.2p1;
- Reproducible on OSX 10.12.3 and Windows 10;
- Reproducuble only when terrain size is extremely big; not reproducible for small terrains.