in the case of an upstream element proposing a buffer pool,
use it to allocate the buffer image with the given parameters
set by the upstream element.
Besides the buffer pool handling is sync'd with GstBaseTransform
base class.
See the case of vulkanupload ! vulkanh264enc
Co-authored-by: Victor Jaquez <vjaquez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5651>