Fade shadows close to the shadow map border.
Fade shadows close to the shadow map border.

The border of the shadow map is obvious (particularly on rocks) because
shadows slide in and out of view as they enter/exit the shadow map
frustum. The shadows should fade out based on distance from the center
of the shadow map (where the player is positioned).

Bug: Issue 27530394

Tested:
Works on Linux and Nexus 5X.

Change-Id: I0f96be467a087ccdbcad063264f68472bfcb9fb8

google/zooshi,google/zooshi,google/zooshi,google/zooshi,google/zooshi