Currently, videocrop, only negotiates raw caps (system memory) because
it's the type of memory it can modify. Nonetheless, it's also possible
for the element to handle non-raw caps when only adding the crop meta
is possible, in other words, when downstream buffer pools expose the
crop API.
This patch enable non-raw caps negotiation. If downstream doesn't
expose crop API and negotiated caps are featured, the negotiation
fails.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/915>
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