vtfpp: ImageConversion::alphaToDistance: init

[partyvan]

i accidentally invented an extremely niche novel dithering algorithm too oops

putting this out there for style/sanity/etc checks while i come up with test cases and maretf flags

[craftablescience]

Since all of this goes over my head and is relatively self-contained, if you can move the additions here to a new header/cpp/namespace I will merge as soon as you're done without my usual nitpicking and just pretend the files don't exist besides setting up language bindings. This is awesome btw and I'm glad you came back to this and sent it in, thanks!!!

[partyvan]

there we go it's been put in its containment chamber with some snacks -- namespace name and so on seem all right to you?

[craftablescience]

Looks good thanks!