Fix 'paddedimage' and 'setratiosize' image formats

- Fix VersionedImageTest unit test
- Support for $color argument in 'paddedimage'.
- Simplified 'setratiosize' case, since looking at Image->SetRatioSize(),
$width and $height are the only arguments, and required - so the for
retrieving those values from $arguments seems unnecessary.
@ajshort Am I missing anything here? `GD->resizeRatio()` has
a third argument `$useAsMinimum`, but that's never exposed
to the caching filename, right?

See https://silverstripe.atlassian.net/browse/CWPBUG-97