[FFmpeg-devel] Encode user data unregistered SEI (H.264/H.265)

[Brad Hards]

MISB ST 0604 and ST 2101 require user data unregistered SEI messages
(precision timestamps and sensor identifiers) to be included. That 
currently isn't supported. This series adds encoding for libx264,
libx265, hevc_nvenc and h264_nvenc in accordance with
ISO/IEC 14496-10:2020 Section D.1.7 and
ISO/IEC 23008-2:2020 Section D.2.7.

v2 removed the API addition, modifies nvenc to use a dynamic array,
and corrects formatting.

v3 removes the example, and improves memory allocation handling.

v4 returns ENOMEM instead of continuing in the event of allocation
failure, and updates the commit messages (consistent with majority 
usage for the affected codec).

v5 fixes the covering email subject.