ccpr: Fix very small edges

Fixes a bug in the vertex shader backend where a pixel could be hit
twice on very small, axis-aligned edges.

Improves the coverage calculations to be more accurate when dealing with
very small edges.

Bug: skia:
Change-Id: I4bac191695d7b7d73b6eef9df0fca3539503a965
Reviewed-on: https://skia-review.googlesource.com/111323
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
4 files changed