Better ligature support in TTF addon.

[SiegeLord]

See discussion in https://www.allegro.cc/forums/thread/618115 and example in https://github.com/lxnt/ex-sdl-freetype-harfbuzz/blob/master/ex-sdl-freetype-harfbuzz.c. We already depend on Harfbuzz indirectly (I think), so using Harfbuzz shouldn't be a big problem.

Actually, I recall that the windows distribution doesn't have Harfbuzz, but if this stuff is implemented, it'd be worth the effort to add it in.