add support for new maxdpr directive

[jonarnes]

maxdpr is a url directive wich define a maxumum DPR (Device Pixel Ratio) to consider when resizing an image. maxdpr can take values from 1 to 4 (including).

image.png?imgeng=/maxdpr_2 will cap the dpr to 2 even if the device has a a screen supporting dpr of 4, for example.