avcodec/ituh263enc: Make static initializations thread-safe

avcodec/ituh263enc: Make static initializations thread-safe

This already makes several encoders (namely FLV, H.263, H.263+ and
RealVideo 1.0 and 2.0 and SVQ1) that use this init-threadsafe.

It also makes the Snow encoder init-threadsafe; it was already marked
as such since commit d49210788b0836d56dd872d517fe73f83b080101, because
it was thought to be harmless if one and the same object was
initialized by multiple threads at the same time.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>