Vertex clamping for shadow maps causes artifacts when large and scaled up objects are used.

To reproduce:

1. Open the project, attached by the user (C130650.zip)
2. Open the "Test2" scene
3. Observe the "Cube' game object in the scene (it doesn't cast shadow)
4. Lower its z scale and see that shadows begin to appear

Expected: shadows are always shown properly
Actual: for large objects, shadows become weird

Reproduced in 5.5.4p2, 5.6.3f1, 2017.1.0f2, 2017.1.0p3, 2017.2.0b6, 2017.3.0a3