avviddec: Take into account coded_height for pool

Failure to do this would cause the decoders to constantly request a new
bufferpool thinking the height had changed ... whereas it hadn't.

Fixes #95

Part-of: <gstreamer/gst-libav!132>
28 jobs for !132 with backport-mr-131-into-1.18-negotiation-coded in 30 minutes and 8 seconds (queued for 10 seconds)
merge request