* `down_only`: downsample only if one of the input dimensions is larger
than the target
* `up_only`: upsample only if one of the input dimensions is smaller
than the target
* `always`: the original behavior
This change doesn't add related modes like area (@ in ImageMagick) or
minimum width/height (^ in ImageMagick). These can be added if a need
arises.
Bug: webp:405437935
Change-Id: I7752789dce6e3b9c3fb7d6edf63ca5559bb3463c