[xavierlacot]

fix #81

This PR moves the srcset generation from the Twig components into a new SrcsetBuilder service, so it can also be used outside of Twig, for instance in an API Platform normalizer. It is also exposed through a new joli_media_srcset Twig filter.

The joli:Img component now builds the srcset of a variation configured with several pixel_ratios from its name alone, instead of requiring the derived variations to be listed by hand. This can be disabled with autoSrcset="{{ false }}".

When a variation has not been generated yet, its dimensions are now computed from the variation definition, so the srcset keeps its width descriptors and the img tag its width and height, even on a cold cache. Pre-processors declare whether they preserve the pixel dimensions with the new PreProcessorInterface::preservesPixelDimensions() method.

Also, the Twig components now read the dimensions from the cached property accessors instead of loading the whole variation file, and the properties of a missing file are no longer cached for a whole day.