Changed the merge algorithm to properly handle cmap subtables
of type 4 and 12 with platform id and encoding ids of 3/1 and 3/10
respectively. All other subtables are not merged and ignored.
The resulting merged cmap table includes a subtable of format 4/3/1
and a format 12 subtable iff there are mappings outside of the BMP.
If one font has two codepoints that point to the same glyph,
and another font has the same code points pointing to two other glyphs,
keep the behavior where the first replacement glyph is stored in 'locl'
and output the third glpyh (or more) to let the user know that they were
dropped, instead of failing to merge the font.
Fixes#444Fixes#322
