Steps to repro:

1. Open Unity;
2. Create new project;
3. Extract attached package;
4. In the Project Browser go to Assets -> Scenes;
5. Open '3_Gameplay' scene;
6. Launch the scene;
7. Move the Camera gameobject or viewpoint.

Actual result:
z-fighting artifact appears on SpeedTree billboards with Point light (see attached gif video).

Expected result:
z-fighting artifact should not appear on SpeedTree billboards.

Notes:
- Reproducible in 5.5.0b7, 5.4.1f1, 5.3.6p6;
- Reproducible on both OSX and Windows;
- Doesn't repro with Directional light.