Add support for MTSDF atlas (a combination of MSDF and true SDF in the alpha channel)

[miltoncandelero]

It is supported by msdfgen so I think it's just a matter of allowing the flag to be passed directly into it🤔

https://github.com/Chlumsky/msdfgen#console-commands

[miltoncandelero]

I lodked into it and the problem seems to be that the text output doesn't output the alpha channel so we can't make the png file :(

[soimy]

Planned in v3.0