webrtcbin: set-description implementation is not spec-compliant
Our implementation of https://w3c.github.io/webrtc-pc/#set-the-session-description doesn't look correct. For instance, we apply step 6.5 (https://gitlab.freedesktop.org/gstreamer/gstreamer/-/blob/0d04660c5dca53096db1711d7925e1920685b137/subprojects/gst-plugins-bad/ext/webrtc/gstwebrtcbin.c#L6454) before step 6.4 (https://gitlab.freedesktop.org/gstreamer/gstreamer/-/blob/0d04660c5dca53096db1711d7925e1920685b137/subprojects/gst-plugins-bad/ext/webrtc/gstwebrtcbin.c#L6595)
Also, an addition to the spec from a couple years ago, calling set-local-description
without description should either use an internally generated offer, or answer, depending on the signaling state. Currently we reject this case (bad input).