Consider decoupling this crate from rav1e

[FreezyLemon]

The current dependency on rav1e feels a bit awkward. I think it would make more sense if this crate encapsulated the actual scenechange functionality and allowed rav1e to just pull it in, similar to v_frame.

This seems more complicated than with v_frame because some parts use rav1e internals, but I don't think it's impossible.

[shssoichiro]

I've wanted to do that, and have rav1e depend on this instead. It's hard to find a clean way to do it, and the biggest issue is the fact that the parts of rav1e that this uses are also used in other parts of rav1e, e.g. the intra and inter cost estimations are also used during rav1e's encoding process for block type decisions.