Values from subsequent values are guaranteed to be identical (since
poc and nal_unit_type are checked to be the same between slices), so
this doesn't affect output in any way, but does resolve the remaining
reported race conditions (by tsan) in fate-hevc.
In practice, this fixes tsan warnings like this:
WARNING: ThreadSanitizer: data race (pid=25334)
Read of size 4 at 0x7d9c0001adcc by main thread (mutexes: write M1386):
#0 hevc_update_thread_context src/libavcodec/hevcdec.c:3310 (ffmpeg+0x000000b41c7c)
[..]
Previous write of size 4 at 0x7d9c0001adcc by thread T1 (mutexes: write M1383):
#0 hls_slice_header src/libavcodec/hevcdec.c:596 (ffmpeg+0x000000b43a22)
This way they can be reused by other code without including the whole
decoder-specific hevcdec.h
Also, add the HEVC_ prefix to them, since similarly named values exist
for H.264 as well and are sometimes used in the same code.
