assert(has_sequence()) in HeifContext::get_track(). The whole sequence/
track API arrived in v1.20.0: 1.19.8 has no matches for has_sequence,
get_track, m_tracks or heif_context_get_track, no libheif/sequences/ and
no heif_sequences.h, and context.cc is 1572 lines while the upstream
patch targets line 2115. nm -D on the shipped libheif.so exports no
sequence/track symbol at all. Same basis as the six entries above. Note
Debian still lists this open for trixie - the claim rests on the code
evidence, not their triage.