avcodec/h263data, ituh263*: Make initializing RL inter table thread-safe

avcodec/h263data, ituh263*: Make initializing RL inter table thread-safe

Up until now, ff_h263_rl_inter was initialized by both ituh263dec and
ituh263enc; this is an obstacle in making the codecs that use this code
init-threadsafe.

This obstacle is eliminated by only initializing this RLTable from
a single place that is guarded by a dedicated AVOnce.

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>