avcodec/utils: av_register_codec & hwaccel() that work in O(1) time

avcodec/utils: av_register_codec & hwaccel() that work in O(1) time

Its possible to implement this with a few lines less code but it then
would flip the order of the list and require registration of external
codecs to be done first, also it could break user applications due to
this. Thus to maintain ABI this slighty more complex solution is
used.

Reviewed-by: Stefano Sabatini
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>