Nicolas Dufresne authored May 18, 2021 and GStreamer Marge Bot committed May 19, 2021

While so far it worked, we are about to introduce a format that break this
assuming. We have a format which consist of NV12 with alpha, and this format
does not have a direct mapping of the component against their plane indexes.

Fix this by using gst_video_format_info_component() introduced in 1.18 for
this purpose.

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