avviddec: Make sure to use a buffer pool with the correct width/height configured on it for pushing buffers downstream
If downstream does not provide a (usable) pool, we would use our internal
pool. But the internal pool might be configured with a different width/height
because of padding, which then will cause problems if we push buffers from it
Instead create a new pool if the width/height is different.
This prevents crashes with vaapisink and d3dvideosink for example.
Based on the debugging results and discussions with
Nicolas Dufresne <email@example.com>