matroska: default prores fourcc apcn

If there is no codec private data for prores it should default to Apple
ProRes 422 Standard Definition (apcn). Can be tested with
strobe_scientist.mkv from
https://developers.google.com/media/vp9/hdr-encoding

Part-of: <gstreamer/gstreamer!1734>
24 jobs for !1734 with mkv-prores-fourcc in 40 minutes and 42 seconds (queued for 3 seconds)
merge request