This removes the dependency that hardware pixel formats previously had on
AVHWAccel instances, meaning only those which actually do something need
exist after this patch.

Also updates avcodec_default_get_format() to be able to choose hardware
formats if either a matching device has been supplied or no additional
external configuration is required, and avcodec_get_hw_frames_parameters()
to use the hardware config rather than searching the old hwaccel list.