Distance field fixes for Android

Distance field fixes for Android
- Expand glyph size by 2 on each side to compensate for bilerp lookup
- Correct for Adreno tendency to drop entire tile if any pixel has divide-by-0
- Fix blurriness on Adreno by using uv coords to compute gradient instead
  of st coords
- Add faster version for uniform scale

BUG=skia:2173

Committed: http://code.google.com/p/skia/source/detail?r=13955

R=bsalomon@google.com

Author: jvanverth@google.com

Review URL: https://codereview.chromium.org/205343008

git-svn-id: http://skia.googlecode.com/svn/trunk@13958 2bbb7eff-a529-9590-31e7-b0007b416f81