when no horizontal/vertical header table is present, we assume numberOfHMetrics == maxp.numGlyphs. This allows to compile/decompile incomplete fonts, only containing head, maxp, hmtx, loca and glyf (or CFF), that is the bare minimum to temporarily store glyph outline and metrics data for use with sparse/intermediate master TTFs in varLib. See https://github.com/googlei18n/ufo2ft/issues/308