Draw an empty border around glyphs to avoid sampling issues

Bug #6942209

The font renderer was preserving a 1 px border around each glyph to ensure
bilinear filtering would work nicely. Unfortunately, this border was not
set to 0 when glyphs were added in the cache to replace old evicted glyphs.

Change-Id: Ib85afca7ebad5cb63f960dc0e87ae162333dbfe8