cli: expose distancealpha

[partyvan]

this won't build in ci until the version of sourcepp updates (also submodule->fetchcontent when), but putting it out here for scrutiny

[partyvan]

some notes:

• starting to expose edge in create cli just because distance stuff needs it, but it may be useful in future to extend it to pass explicit edge policy into all resize operations done by the program
• VTF::create(filepath,...) appears to support automagically grabbing frames from gif/apng/etc which is wicked cool, but reimplementing a library feature in the tool feels gross, so currently, createPossiblyDistanceMappedVTF does not have full parity at its call site if distance mapping is enabled (vanilla case still passes through to the original function so thats fine); i compromised by just adding, for now, a possibly-distance-mapping replacement for setImage in the flat-animation-from-numbered-filenames case. even if it's limited for now, being the first source texture tool capable of generating animated distance maps is gonna be neat!!
  • the easiest option i see to make this cleaner would be variants of VTF::create that accept a callback operating on image data. that way we could get animated distance maps from apngs as well :D not to mention any other post-processing that library users may wish to account for
    • this may in turn call for the library to start using a lightweight image data representation that carries dimensions alongside data to remedy the pervasive outparam game of telephone

[partyvan]

seems stable after a bunch of local usage; rebased onto a module bump so it builds here

[partyvan]

occurs to me that it probably actually needs craftablescience/sourcepp@aa9558d in order for pipelines to run (this was actually the motivating example behind that patch moreso than building the library itself)

also crosslink bookkeeping
supersedes: #25
addresses: #19

[partyvan]

rebased onto mane & resolved conflicts