Font fallbacks

[dhardy]

@RazrFalcon have you thought about font fallbacks? As I understand, there are two parts: selecting fallback fonts, and building a virtual font pulling glyphs from the first supporting font. The second part could be supported here or could be left to a higher-level library.

The Face struct has several classes of methods:

• from_slice constructor
• has_table, table_data, character_mapping_subtables, ... — low-level stuff which it doesn't make sense to expose in a virtual "fallback font"
• is_regular, is_monospaced, weight, ascender, ... — properties which could be taken from the first font (or perhaps in some cases combined via a func like max)
• glyph_index(char), glyph_hor_advance(glyph_id), outline_glyph(glyph_id, builder), ... — glyph-specific functionality abstracted over by the virtual fallback font

The GlyphId type currently uses a u16 which is likely insufficient for the fallback font — this type could be made an associated type of a trait or simply larger.

Given the above, it might make more sense to implement font fallback at a higher level, e.g. via the ab_glyph::Font trait (if @alexheretic wishes to support font fallbacks), but I thought it would make sense to ask here first since (a) you (@RazrFalcon) may have already considered this and (b) whatever choice is made here has implications for downstream libraries (e.g. whether GlyphId becomes an associated type).

[RazrFalcon]

Font fallback is out of scope of this library and will probably be implemented in a separate crate using the fontdb. Also, I'm planing to use the shaper-based fallback. But I have to finish porting harfbuzz first.

I've actually have a sort of roadmap here. And as you can see, we're are miles behind. At least in a pure Rust way.

The GlyphId type currently uses a u16 which is likely insufficient for the fallback font

TrueType glyphs are always u16. So this is not a problem. And there is no point in supporting other font formats.

Anyway, this crate will remain just a low-level, zero-alloc TrueType parser. Nothing more.