Fix artifacts in distance field rendering due to bilerp

Fix artifacts in distance field rendering due to bilerp

Because we were using the full distance field glyph rect, there were cases
when a neighboring texel might be set to full value (e.g. we might copy the
glyph over old data after a purge). This was giving artifacts, and insetting
the rect by 1 solves the problem. In doing this, I discovered that removing the
extra 1 texel pad around the glyph meant to handle bilerp, and insetting by 2,
works just as well and saves space in the glyph atlas.

R=bsalomon@google.com

Author: jvanverth@google.com

Review URL: https://codereview.chromium.org/239333002

git-svn-id: http://skia.googlecode.com/svn/trunk@14203 2bbb7eff-a529-9590-31e7-b0007b416f81