plugin: if any caps in downstream, negotiate raw video

When downstream has any caps, vaapi should not shovel vaapi featured buffers, but rather plain raw video, assuming always the worst case scenario (downstream cannot handle featured video memory but raw system memory buffers).

This patch query the peer caps without any filter, to know if donwstream just ask for any caps, if so jump to the color space checking, otherwise do the caps intersection and continue with the feature selection algorithm.

Fixes: #139 (closed)