mfvideosrc: Fix for negative MF stride

Negative stride value can be used in MediaFoundation to inform
whether memory layout is top-down or bottom-up manner. Note that
negative stride is allowed only for RGB, system memory.

See also

Fixes: gstreamer/gst-plugins-bad#1646
Part-of: <gstreamer/gst-plugins-bad!2477>
29 jobs for !2477 with backport-mr-2473-into-1.18-mfvideosrc-negative-stride in 36 minutes and 6 seconds (queued for 2 seconds)
merge request