This implements dynamic font protobuf generation, allowing users to
request font ranges on the fly, and combining them in any order, e.g.
`Font1,Font2,Font3`, same as with sprites and tiles
This is a first iteration, without any multithreading support. In
theory, this could be done far faster by generating SDFs with multiple
threads.
### Current process
* during init, figure out all glyphs available in each font, and store
them as a bitset
* during request:
* combine requested bitsets to figure out which glyph should come from
which font file
* load those glyphs from files (using a single instance of the freetype
lib)
* convert them to SDFs and package them into a protobuf
---------
Co-authored-by: Lucas <zhangyijunmetro@hotmail.com>
