video: Fix scaling in 4x horizontal co-sited chroma
4x downscaling of chroma with co-sited chroma has never worked it seems.
Fixes incorrect videotestsrc output and videoconvert conversions to Y41B, YUV9, YVU9 and IYU9 with co-sited chroma.
e.g.
gst-launch-1.0 videotestsrc ! video/x-raw,format=Y41B,width=1280,height=720 !
videoconvert ! autovideosink