Add support for glyph index based msdfgen-ext APIs

[KitsuneAlex]

This PR adds support for the following msdfgen-ext functions:

• msdfgen::loadGlyph(msdfgen::Shape, msdfgen::FontHandle, msdfgen::GlyphIndex, int) for loading glyphs by their glyph ID directly instead of their codepoint
• msdfgen::getGlyphIndex(msdfgen::FontHandle, unsigned) for retrieving the glyph index of a given codepoint
• msdfgen::getKerning(double&, msdfgen::FontHandle, unsigned, unsigned) for retrieving the kerning between the two given unicode codepoints
• msdfgen::getKerning(double&, msdfgen::FontHandle, msdfgen::GlyphIndex, msdfgen::GlyphIndex) for retrieving the kerning between the two given glyphs referenced by their indices

This allows using this library in conjunction with other libraries like harfbuzz.