[FFmpeg-devel] Add user data unregistered SEI writing for libx265

[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 for libx265. This patch adds support
for user data unregistered SEI messages in accordance with 
ISO/IEC 23008-2:2020 Section D.2.7

The design is based on nvenc, with support finished up at
57de80673cb