Masks manipulators

[lainosantos]

Many projects needs create, manipulate, cache and serve images with a mask because design (ui) decisions. A very commom mask is circle, for image profile, as example, or for a PWA icons sizes and formats.

A pull request was rejected in past (#104) in favor of css border-radius propertie, but in some cases the css isn't possible.

A Glide bultin support for basic masks is a great feature, like https://www.imgix.com/ does.