smartencoder: Fix renegotiating when reencoding parts of the stream with vpx

smartencoder: Fix renegotiating when reencoding parts of the stream with vpx

In the encoded streams we might not have all the information about the
raw video stream, but when reencoding they end up being specified, even
if those are default values.

As vp8 decoders always output frames in some YUV color space we can
ensure that when upstream doesn't specify any value in its caps we
use the default one which is what we end up doing when decoding/reencoding
anyway, so this way downstream (matroskamux in that case) doesn't need
to be able to renegotiate (which it doesn't).

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1062>