Include previously registered sizes in `filter_image_downsize()`

[peterwilsoncc]

The downsizing filter uses Tachyon::image_sizes() to determine valid sizes for an image to account for image sizes registered after an image was uploaded.

If registered sizes were removed after an image was uploaded, the legacy size will be referenced in wp_get_attachment_metadata( $attachment_id )['sizes']. To account for these, the downsize and content filters should combine both the meta sizes with registered sizes.

Design decision: should a clash exist between the legacy and current image size names, which version should be deferred to?