rtppayload: Fix VP8/VP9/OPUS dual encoding name handling

All these were copy pasted and would lead to assertion when chained with
rtpmux. This commit rewrite the negotiation with downstream. This also
drop the fallback to ancient names if the pad is unlinked. This was
completly arbitrary decision that made no sense.

https://bugzilla.gnome.org/show_bug.cgi?id=796809
3 files changed