Support typographic features

[jminer]

It should be possible to turn on typographic features in a font.

For DirectWrite, the GetGlyph() function takes an array of DWRITE_FONT_FEATUREs.

For Core Text, it looks like you have to create another CTFont with the features enabled:

https://stackoverflow.com/questions/59464935/uifont-how-to-use-stylistic-alternate-character

I could probably do the same, and have the enabled features in the Font struct. But I think the DirectWrite API is better. I could easily implement the Core Text API using DirectWrite, but I couldn't implement the DirectWrite API using Core Text as easily, so I think the DirectWrite API is better. Pango's API seems closer to DirectWrite's. You can apply a font features attr to an item returned from pango_itemize() before passing it to pango_shape(). It even gives font features as an example:

https://docs.gtk.org/Pango/method.Item.apply_attrs.html

Perhaps it would be best to have the TextAnalyzer::get_glyphs_and_positions() method take a Vec<FeatureRange> or something. Not sure.