JPEGDepay not working if rtpmap attribute is missing

[heppth]

Can not depay MJPEG RTSP stream if SDP message does not contain a rtpmap attribute.

Reason
The VideoMedia can not be found, because rtpmap is undefined

media-stream-library-js/streams/src/components/jpegdepay/index.ts

Lines 23 to 28 in 07f9471

	const jpegMedia = msg.sdp.media.find((media): media is VideoMedia => {
	return (
	media.type === 'video' &&
	media.rtpmap !== undefined &&
	media.rtpmap.encodingName === 'JPEG'
	)

...if camera sends DESCRIBE response like this:

v=0 
o=- 1674614617067403 1 IN IP4 192.168.40.45 
s=Mobotix IP-Camera (MJPEG, unicast) 
i=stream0/mobotix.mjpeg 
t=0 0 
a=tool:LIVE555 Streaming Media v2018.10.17 
a=type:broadcast 
a=control:* 
a=range:npt=0- 
a=x-qt-text-nam:Mobotix IP-Camera (MJPEG, unicast) 
a=x-qt-text-inf:stream0/mobotix.mjpeg 
m=video 0 RTP/AVP 26 
c=IN IP4 0.0.0.0 
b=AS:22500 
a=control:track1 
a=framesize:96 800-600 
a=x-dimensions:800,600 
a=recvonly 

Suggested fix
Do not require rtpmap and check fmt instead. For example:

          const jpegMedia = msg.sdp.media.find((media): media is VideoMedia => {
            return (
              media.type === 'video' &&
              media.fmt === 26
            )

...because the RFC says:

In the case of RTP streams, all media descriptions SHOULD contain
"a=rtpmap" mappings from RTP payload types to encodings. If there is
no "a=rtpmap", the default payload type mapping, as defined by the
current profile in use (for example, RFC 1890 [5]) is to be used.

[steabert]

Since the server should send it, this isn't high prio. However, it would be nice to have a fallback for servers that don't do what they should do.

For the fallback, I don't think we can use the suggested fix, as we read the payload type later on. So we would need a way to completely bypass payload type selection and go for the default 26.