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>