vtdec specifies sysmem; GLMemory as template caps. When negotiating, we used to
call gst_pad_peer_query_caps (..., filter) with our template caps as filter. The
query does gst_caps_intersect (filter, peercaps) internally which gives
precedence to the order of the filter caps. While we want to output sysmem by
default, when negotiating with glimagesink which returns GLMemory; sysmem; we
do want to do GL, so we now query using a NULL filter and intersect the result
with our template caps giving precedence to downstream's caps.
tl;dr: make sure we end up negotiating GLMemory with glimagesink
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