Use the first payload type number for each codec in the SDP answer

It is possible for the SDP offer to feature more than one payload type
number (either static or dynamic) that refer to the same codec.  For
example, a peer that supports ITU-T V.152 may use two payload types,
with one of them marked for voice-band data use:

    m=audio 3456 RTP/AVP 0 98
    a=rtpmap:0 PCMU/8000
    a=rtpmap:98 PCMU/8000
    a=gpmd:98 vbd=yes

Ideally, Twinkle should use the number that was listed first in the
media formats list when building the SDP answer.

Closes #184

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