Despite the name, we were passing FrameInvariants to encode_frame()
 as &mut.
There were two places that it actually mutated the contents.
1) In write_sequence_header_obu(), several fields were set to
    constants.
   Given that we write the sequence header multiple times, but its
    contents should never change, the initialization does not
    belong in this function, as that is just asking to violate this
    constraint if we ever make them non-constant.
   Instead just initialize the fields when the rest of the
    FrameInvariant struct is initialized, since the values are
    currently constant anyway.
   We can find a better place than write_sequence_header_obu()
    (one that really is called once per sequence) if we ever want
    to make them non-constant.
2) The ref_frame_sign_bias[] fields are initialized at the very
    start of encode_frame().
   These can instead be initialized in new_inter_frame() as soon as
    ref_frames[i] is set.

With the above two changes we can drop the &mut and ensure that
 FrameInvariants really is invariant.