mac-videotoolbox: Set default keyframe interval to 2 seconds

0 seconds means "auto" according to the documentation, but this appears
to be broken in many configurations (more than just CRF mode on Apple
Silicon). With some encoders it means that the encoder sets a keyframes
every 31st frame, other times it just doesn't set any keyframes at all
after the first one, only rarely does the "auto" interval actually
appear to work.
Lets just set the default to 2 seconds and be happy. In theory this is
the maximum keyframe interval and encoders are allowed to set more if
they so wish, but they never appear to do so.