openh264dec: The codec can only do baseline so simplify the code a bit

and declare this also on the sinkpad caps.

With baseline profile there are no B-frames or frame reordering.
openh264enc: make denoise work

https://bugzilla.gnome.org/show_bug.cgi?id=737894
openh264: Use a MARGINAL rank for the encoder and decoder

Once we start automatically selecting decoders the rank
becomes important.