Remove support in GPU backend for distance vector field.

Also, remvoes SkNormalBevelSource as this was the last use case for the distance vector field.

Change-Id: Ib0176c78e500e6b5130310934253a75860245812
Reviewed-on: https://skia-review.googlesource.com/18482
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
34 files changed