More fixes for distance field paths

Disables use of SDFs for very small paths (because of blurring) and
adds a border of 1 pixel in device space to handle antialiasing.

BUG=chromium:677889

Change-Id: I81e49477c943d41523fd836e55abd696a985491f
Reviewed-on: https://skia-review.googlesource.com/6832
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
3 files changed