Support custom character to glyph mapping

[jminer]

I'm not sure I care enough to support custom character to glyph mapping, but it should be possible to do cross-platform. However, Core Text is really different than DirectWrite.

With DirectWrite, it's possible to modify the glyph array that GetGlyphs() returns before passing it to GetGlyphPlacements().

For Core Text, you can create a CTGlyphInfo object and store it in a attributed string using kCTGlyphInfoAttributeName, as this answer says:

https://stackoverflow.com/questions/20327980/getting-glyph-names-using-core-text