It doesn't make sense to define onTranscodingProgress inside of mediaProcessConfig. Instead it makes sense to define onTranscodingProgress callback at callsite and pass into mediaProcessConfig. This also makes mediaProcessConfig more generally useful.
I initially added the onTranscodingProgress stuff to InputStateContainer so my bad.
(I'm NOT planning on doing any more refactoring here. This was confusing and 2 min change so I just made it.)