Refactoring extractMTParams

[rbgodwin]

This issue captures observations from code reviews performed as part of extending sdpoker to support ST 2110-22 (JPEG-XS). During the review it was observed that extractMTParams could do with refactoring

• Ought to split on m= and then produce one entry for each media description,
• currently files with e.g. two a=fmtp: lines per media description, or none (some media types don't need one), or media descriptions where the a=rtpmap: line comes after the a=fmtp: line rather than before, will produce unexpected results