Better handling of srcset and sizes

@deepali sardana : thanks for the merge Request, but I'll close it, because there is a lighter way to acheive that.

drdam → closed merge request !2

drdam → changed the visibility of the branch 3496234-better-handling-of to active.

In fact, this case is not natively covered by nativ responsiveImage because of the possible mixe between "allow/no-allow" upscaling.

The problem is that the imageStyle stack does not provide a simple way of finding the ‘expected size of the derivative if the source image is large enough not to be upscaled’ for an image style.
In fact, it is not possible to ‘classify’ image styles by this ‘expected size’ and therefore to find the ‘smallest’ image style that is most suitable.

The anomaly is present in native ResponsiveImages too.

Exemple as a native src-set image :

The "tiniest" size ( 100px) better fit for the thumbnail image style are associate with the "wide" image style because it's the "last" of the list.

I have rework the "association loop" for trying to manage mixte between not-upscaling / upsaling.

The association between size and imageStyle are "not optimal" because of #10 and #11, but it doesn't really matter because the derivative will be the same, just not in the "best folder" on the disk

fixed in 1.0.0@alpha2