vp8decoder: fail early if no input caps have been provided

vp8decoder: fail early if no input caps have been provided

The vp8decoder class does not implement the ->parse() virtual function,
it can only accepts frame aligned data. If some element such as filesrc
feed it with unaligned data, the behaviour is undecided. So we should
set_needs_format of the decoder to TRUE, then it can fail with a
"not-negotiated" error early, rather than go on and generate unexpected
error.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4066>