Use highp for distance field texture coord varyings.

Because the glyph texture atlas is 1024x2048, on certain platforms
using mediump UVs is not enough resolution for doing texture lookups
and getting good results for distance fields. Bumping these
to highp solves this problem.

BUG=skia:3445

Review URL: https://codereview.chromium.org/951003002
1 file changed