vaapipostproc: resize if negotiated and allocation caps are different

vaapipostproc: resize if negotiated and allocation caps are different

Since commit 859a2b2, in vaapidecode, allocation query can be different from
the negotiated caps.

When connecting the vaapidecoder to the vaapipostprocessor, the last one will
resize the frame to the negotiated, if and only if, some other parameter is
activated to avoid the passthrough. If it is not, the surface won't be mapped
into a image. If not, the image won't be resized and the output buffer would be
mapped.

This patch will break the passthrough if the allocation query is different
from the negotiation caps, forcing the resizing.

https://bugzilla.gnome.org/show_bug.cgi?id=765095