avcodec/mpegvideo: Deprecate bframe opts for encoders not supporting them

avcodec/mpegvideo: Deprecate bframe opts for encoders not supporting them

MPEG-1/2/4 are the only mpegvideo based encoders that support bframes;
yet even the encoders not supporting bframes have options that only make
sense for an encoder that supports bframes; setting any of these options
for such an encoder has no impact on the encoded outcome (but setting
b_strategy to two slows down encoding considerably). So deprecate these
options.

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