Don't scale chroma subsampled images to sizes that don't align with the subsampling...

Don't scale chroma subsampled images to sizes that don't align with the subsampling (#1872).

There's a slightly odd effect when scaling e.g. YVU420 images to
odd and then even widths - there's a small but visible luminance
shift.  I don't know why this happens, but keeping the scaling
sizes locked to the subsampling seems to help.