viddec: avoid renegotiation

viddec: avoid renegotiation

Keep track of the context veriables and renegotiate when something changes. We
need to use separate values for this because not all values can be copied into a
GstVideoInfo.
Set the output interlacing based on the libav context.
Use the demuxer and decoder framerate to construct an output framerate.
Use demuxer and decoder par to construct an output framerate.