The SEI handling of libx265 is buggy and can easily lead
to memory corruption: It reuses certain buffers, but when
reusing them it presumes that it is enough for these buffers
to exist and does not check whether they are actually large
enough to hold what is intended to be stored in them.*

Our users are exposed to this because forwarding A53 CC data
is enabled by default. Change this to make it disabled
by default.

"Fixes" tickets #10411, #11052 and (presumably) #10906.

*: See https://trac.ffmpeg.org/ticket/9666#comment:1Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>