struct refmvs_frame: Make fields safer without changing layout

[randomPoison]

Also cleanup rav1d_refmvs_init and rav1d_refmvs_clear.

• frm_hdr - Pointer to f.frame_hdr. We may be able to pass frame_hdr into the places where it's used. frm_hdr is only used from rav1d_refmvs_find and add_temporal_candidate, which is only called from rav1d_refmvs_find. So if we can pass f.frame_hdr into rav1d_refmvs_find and then down into add_temporal_candidate we should be able to remove this field. Since frm_hdr is not used by load_tmvs_c (and thus not accessed by asm), we should change it to a *const (), signifying that it should never be used while keeping the layout the same. It is dangerous to keep it as a *const Rav1dFrameHeader, since it's a *const Dav1dFrameHeader in C. And passing the &Rav1dFrameHeader through the callstack removes the lifetime issue as well.
• rp - Points to f.mvs. rp is also not accessed in load_tmvs_c, so hopefully we can change it (in a layout preserving manner).

[fbossen]

There may be a conflict with #821 where a pointer to a struct refmvs_frame is passed to an asm function.

[randomPoison]

Ah, that's a good point. Even before #821 refmvs_frame is being passed into asm functions, so I'm not sure we can actually make changes to it safely. The sister struct refmvs_tile seems to only be used from Rust so I think we can still clean that one up.

[kkysen]

I think we should do part of this. At least, frame_hdr should be made a *const () and passed separately as a reference through the callstack as @randomPoison proposed above. It seems dangerous to leave a *const Rav1dFrameHeader when C has a *const Dav1dFrameHeader. We can make refmvs_frame safer; we just can't change its layout at all, and we can't fields that are accessed in load_tmvs_c (and thus what's accessed by asm). See also #821 (comment).

[kkysen]

• Fixed by struct RefMvsFrame::{rp,rp_ref}: Remove raw ptr fields and pass through the callstack #983.