Closes #503.

I've got some comments in there with questions about how to do certain things.

I had thought I could use hb_font_set_variations and it would be super easy,
barely an inconvenience, to support setting variations but it does nothing at all.
Instead I had to use FreeType directly, which took me a while to figure out how
to do. Do you think hb_font_set_variations is supposed to work? Or is it correct
that it does nothing when using FreeType, perhaps?

font->enable_kerning is not used at all anymore, TTF_SetFontFeatures and TTF_SetFontKerning
are completely independent. It ignores the font->enable_kerning value
if TTF_SetFontFeatures has been called with a non-null string. If TTF_SetFontFeatures
is called later with a null string, it starts using the font->enable_kerning value again.
Combining them is a bit complicated if we want to avoid doing more work in CollectGlyphsFromFont().

I've tried to run address sanitizer on the code but I haven't got it working yet.
It reports nothing even though I've commented out calls to free.

Until I've figured out how to get ASan working, and if hb_font_set_variations is supposed to work, I'm keeping this as
a draft PR