avcodec/nvenc: Add support for H.265 encoding

avcodec/nvenc: Add support for H.265 encoding

Sufficiently new nvenc hardware (GM20x or later) has support for H.265
encoding. This works the same as the H.264 encoder except the
codec parameters are different.

Due to the fact that common codec parameters are not shareable, there's
quite a bit of conditional logic you'd wish we could do without, but
such is life.

Signed-off-by: Philip Langdale <philipl@overt.org>
Reviewed-by: Timo Rothenpieler <timo@rothenpieler.org>