ffv1enc: write f->flt to extradata

The parser has been reading f->flt for combined_version >= 0x40004
since commit c1b330bf24 (avcodec/ffv1: Basic float16 support), but
ff_ffv1_write_extradata() never had a matching put_symbol().
The result was that the parsed f->flt was whatever the next symbol's
worth of rangecoded bits happened to decode to — often 0, but for a
yuv420p16le -level 4 -strict experimental stream produced locally it
parses as 1.  The software decoder doesn't notice because the YUV
pixfmt-selection branches never check f->flt, but anything else that
trusts it gets garbage.

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