VA-API based buffer might need a video meta because of different
strides. But when donwstream doesn't support video meta we need to
force the usage of video meta.
Before we changed the buffer pool configuration, but actually this
is a hack and we cannot rely on that for downstream.
This patch add a check fo raw video caps and allocator is VA-API,
then the option is enabled without changing the pool configuration.
In this case the element is responsible to copy the frame to a
simple buffer with the expected strides.
https://bugzilla.gnome.org/show_bug.cgi?id=785054
