custom codecs conflict with default ones

[brzep]

I came across this issue while developing an app using webrtc-rs. After creating MediaEngine, I call register_default_codecson it. Then I callregister_codec` twice, registering codecs that I use in my app, with custom payload types.

When I was testing a connection with one service, it consistently failed, signaling an invalid sdp offer. It turned out, that there was more than one line that was mapping payload type to certain codec, eg. a=rtpmap:96 H264/90000. To fix this I just stopped registering default codecs, because it doesn't make any sense for me to do this anyway.

Shouldn't the library prevent such behavior? Maybe throw some error when registering conflicting payload types?