Fix fontconfig issues connected to sandboxing

Fix a bug in fallback font rendering with cairo

The freetype library and the font face must be keep open for as long as
cairo uses it.

Fix offsets in text rendering, release freetype resources on error and
switch themes.

Merge branch 'master' of github.com:johanmattssonm/birdfont

Fix overview color and glyph alignment

Use roboto as the default fallback font

Scale all glyphs to the same size in fallback font

Faster rendering of fallback fonts in the overview tab