Per the spec[1], the bounding box should be calculated and if it differs
from the one stored on the font, the original should be written
otherwise it should be omitted. On the other hand, the bounding box of
composite glyphs should be always written.
Fixes#12.
1. http://dev.w3.org/webfonts/WOFF2/spec/#conform-mustCalculateOmitBBoxValues