Navmesh preview at different camera distances and angles is displayed differently.

Reproduction steps:
1. Open attached project
2. open the scene "test"
3. Open the Navigation window
4. Try to zoom in and out while looking to terrain in scene view
5. Observe how navmesh preview changes depending on the zoom level

Reproduced on: 5.5.1f1, 5.6.0f3, 2017.1.04

Won't fix, because we don't have the resources to solve z-fighting for debug graphics