Review the cmap compaction code

[bstell]

Review exactly what is in the compacted cmap data in the base font.

[bstell]

When there is only a format 4:

• is the cmap compacted?

When there is both a format 4 and 12:

• are the two merged
• (for future cmap rebuilding: is format 4 idRangeOffset disabled?)

[bstell]

Force and assert idRangeOffset is not used. Then
cmap_compacter:_GOSGenerators:type4 - should be a calculation for format 12

[bstell]

perhaps could do a sparse cmap:

• make a segment per codepoint
• repeat a single mapping (would compress well)
• to make fixup faster put in a real value every N (say 256) values
  -- would require the glyph be included; every 256 => 0.4% => 4M -> 16K
  -- would want the header to list which of the chars have glyphs