Added in support for fallback font groupings.

[adamharrison]

Added in support for font groupings. Basically just reused the font metatable, but if on a table, will use those fonts as fallbacks if a glyph can't be found in the primary font. If a glyph still can't be found, will draw U+25A1; the white square.

Should close #591 .
Should close #611 .

Thoughts? I will likely be slimming this down if I can further.

[adamharrison]

For reference, sample usage is like so:

syntax.code_font = renderer.font.group { font1, font2, ... } 

[adamharrison]

Should close #518 .

[adamharrison]

Hey; I'm going to target this for a merge on the 20th, if no one has any objections. It's a fairly self-contained change, that shouldn't affect any existing code, so hopefully it's non-controversial enough to just merge in.