Add variable font support for SFD sourced projects

[alerque]

The machinery to generate variable fonts from FontForce sources (SFD) isn't here yet. It's possible to do with the addition of a designspace file and on the fly conversion to UFO (which we need anyway for exchange with other contributors), we just need to wire it up to be usable. The end user shouldn't have to worry about more than clean sources with the right names and layout plus a designspace that maps them.

An existing variable font project drawn and built from SFD would be a good place to start. I can mock one up but if there is an open source project out there already we can convert as a proof of concept that would be spiffy.

[alerque]

Some useful hints in a demo from @ctrlcctrlv: https://youtu.be/xoQuWARCUWI

[ctrlcctrlv]

@alerque https://github.com/ctrlcctrlv/Noto-Sans-Tagalog is a real font built using that method, FYI :-)

[alerque]

Excellent! Thanks Fred, that will give me something to play with that doesn't look like a horror film workshop. My own SFD font projects are not up to this yet.

[ctrlcctrlv]

And then there were two! 😄

https://github.com/ctrlcctrlv/Noto-Serif-Tagalog