plugins: set negotiation caps in src allocator
plugins: set negotiation caps in src allocator

When the allocator is created, it stores the allocation caps. But sometimes
the "allocation caps" may be different from the "negotiation caps".

In this case, the allocator should store the negotiation caps since they
are the ones used for frame mapping with GstVideoMeta.

When vaapispostproc is used, this is not a problem since the element is assume
to resize. But when using a vaapi decoder only, with a software renderer, it
fails in this case.

https://bugzilla.gnome.org/show_bug.cgi?id=773323

ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi