[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.

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