Steps to repro:

1. Open attached project ad 't1' scene;
2. Select 'Main Camera' gameobject;
3. In the Inspector drag Camera's Rotation Y value.

Actual result:
There are banding artifacts around the spotlight cookie. See attached video.

Expected result:
Spotlight cookie should be rendered without banding artifacts.

Notes:
- Reproducible in 2018.2.0a1, 2018.1.0b3, 2017.3.0p2;
- Reproducible on Windows 10;
- Reproducible in both Editor and Standalone Player;
- Reproducible only when the light is set to realtime.