Allow Specifying RAPs

[dwbuiten]

Not sure if this belongs here, on the crav1e repo, or both.

We need a way to specify whether or not a given frame passed into rav1e (via API) will be encode as a RAP or switch point (be it a keyframe, S-frame, or whatever).

This is useful because we want, for example, the encoder to place these sorts of frames whre it sees fit (at scene changes, where it makes sense cost-wise), but also at specified intervals as well, or at chunk boundaries, etc.

For some reason crav1e doesn't have a struct for frame properties, but instead has getter/setter functions, so I supposed it would have to be something like:

void rav1e_frame_set_type(RaFrame *frame, RaFrameType type);

[lu-zero]

On this side to have the feature, on crav1e to have it exposed.