videoconvert: Increase rank to secondary.

Most of video filters has rank none, so when autovideoconvert is used,
the alphabetical order is used, where this element might be the last
one, and this is not the desired behavior because videoconvert
normally is more stable.

This patch increases the rank of videoconvert so it could be chosen
before others. If some user prefer other color converters they can use
GST_PLUGIN_FEATURE_RANK environment variable.
37 jobs for !1161 with videoconvert-increase-rank in 33 minutes and 36 seconds (queued for 2 seconds)
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