sdp: Improve h.264 caps <-> media accuracy
We had API to convert a SPS to profile and level identifiers, but not the
reverse operation. This new function can be used for instance when generating
SDP messages including profile-level-id
attributes for a given H.264 caps.
sdp: Generate profile-level-id attribute for H.264 caps
The attribute value is the SPS generated from the caps `profile` and `level`
fields, exported as a string.
sdp: Extract level from H.264 profile-level-id attribute
The H.264 caps generated from a H.264 SDP media now represent more accurately
the SDP media.