qtdemux: add depth for ProRes 4:4:4:4 variants if available

Might be 24bpp in case an alpha channel is coded but
the image is always opaque.

Part-of: <gstreamer/gst-plugins-good!1062>
32 jobs for !1062 with backport-mr-1061-into-1.18-ruslank-master-patch-11248 in 41 minutes and 49 seconds (queued for 4 seconds)
merge request