Handle AVID MJPEG streams directly in the MJPEG decoder.

Handle AVID MJPEG streams directly in the MJPEG decoder.

AVID streams - currently handled by the AVRN decoder - can be (depending
on extradata contents) either MJPEG or raw video. To decode the MJPEG
variant, the AVRN decoder currently instantiates a MJPEG decoder
internally and forwards decoded frames to the caller (possibly after
cropping them).

This is suboptimal, because the AVRN decoder does not forward all the
features of the internal MJPEG decoder, such as direct rendering.
Handling such forwarding in a full and generic manner would be quite
hard, so it is simpler to just handle those streams in the MJPEG decoder
directly.

The AVRN decoder, which now handles only the raw streams, can now be
marked as supporting direct rendering.

This also removes the last remaining internal use of the obsolete
decoding API.