Before this patch, there could be duplicate payload types in offers that
have, within a media section, multiple codecs and RTX enabled:
```
m=video 9 UDP/TLS/RTP/SAVPF 96 97 97 <-- HAS DUPLICATES
a=sendrecv
a=rtpmap:96 VP8/90000
a=rtcp-fb:96 nack
a=rtcp-fb:96 nack pli
a=rtcp-fb:96 ccm fir
a=rtcp-fb:96 transport-cc
a=rtpmap:97 H264/90000
a=rtcp-fb:97 nack
a=rtcp-fb:97 nack pli
a=rtcp-fb:97 ccm fir
a=rtcp-fb:97 transport-cc
a=rtpmap:97 rtx/90000 <--------- PT IS DUPLICATE
a=fmtp:97 apt=96
```
Fix this by populating the media_mapping array with all media formats
rather than only the first one. The added test case reproduces the issue,
which fails without this patch.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8259>
b83b73841d