Erroring out of NVENC init early if the Max B-frames setting was higher
than the encoder's capability causes an encoder failure on NVIDIA Pascal
(10-series) and earlier GPUs due to an unfortunate interaction between
Simple Output Mode, HEVC, and our default B-frames setting of 2. Since
we already check the Max B-frames capability of the encoder, cap at that
value instead of erroring out.
Fixes#7698.
f405647419