Interested in porting the font-rs rasterizer to FreeType

[Tatsuyuki Ishi]

Hi everyone,

I've been tinkering with vector rasterization recently and I think
I've learned enough to do the font-rs porting task from the GSoC ideas
page. Just to be clear, I'll be doing this as my free-time
contribution, not as a part of GSoC.

For the porting, I currently have these questions:
* I plan to do a rewrite in C due to 1. dealing with build systems is
annoying and Rust/Cargo integration with other build systems is
immature, 2. the font-rs rasterizer code is rather simple (about <200
lines), and 3. font-rs is not polished enough for production (for
example lacking proper handling of out-of-bound coordinates). Just for
a reference, someone at Google have ported the same code to Go:
https://github.com/google/font-go. Does porting sound good in this
case?
* Should the old rasterizer be retained? Probably yes, because the
FT_RASTER_FLAG_DIRECT thing is coupled with a FT_Span structure that
is not so much compatible with font-rs's idea (it uses a dense prefix
sum structure rather than sparse spans). If we are going to keep the
old renderer, then should we also provide a build-time option that
allows defaulting to the old engine?

Tatsuyuki