Responsive images that support more than one media format

it's merged

PR looks good to me! - i've only some rather cosmetic comments.

I fix the problem as follows:

• MediaFormatResolver generates additional width-based media formats for all original media formats, not only for the first one. These media formats have the property parentMediaFormat which contains the original media format.

• MediaSource::resolveAllRenditions tries first to find renditions that match only the original media formats. Only when this was successful - that means any rendition is found and all mandatory renditions are resolved - it goes further and tries now to resolve the additional media formats of each of the original formats.

So the parent-child relationship is solved via mediaformat properties and we don’t need to touch the API and have no problem with backward compatibility.

Please note that the whole logic applies to image sizes only because picture sources have each exactly one mediaformat and its own widths. So they don’t need this parent-child relationship.

PR: https://github.com/wcm-io/wcm-io-handler/pull/17

To solve the problem, the additional media formats (for responsive widths) should be generated for all media formats, not only for the primary one. But if we put them all in a flat list, the asset must fit to all combination of original media formats and the additional ones, which is very unlikely. We should rather put the additional media formats into a sublist of the original format. That means if the asset matches the original media format, it should also match the additional width formats related to the original one.

Example: suppose we have

• mediaformats: mf1, mf2

• widths: w1, w2, w3?

The rendition check must be as follows: the asset must match at least one of the following combinations:

• [mf1, mf1_w1, mf1_w2, mf1_w3?]

• [mf2, mf2_w1, mf2_w2, mf2_w3?]

I guess this requires refactoring of the MediaArgs class, so that instead of having one array of MediaFormatOptions, we have an array of arrays of MediaFormatOptions, or a map of original media format to additional ones <mf1: [mf1_w1, mf1_w2, mf1_w3]>, <mf2: […]>, …

An alternative solution is to put all media format combinations in a flat list in MediaArgs and only in MediaSource::resolveAllRenditions separate them in different lists, which are checked independently. This is a bit hacky because we should detect the related mediaFormats with a regexp like <string>__<digits> but has the advantage that the API of MediaArgs remains intact, which is good for backward compatibility.

what do you think? any better ideas?