Derivatives should not be created on read-only stream wrappers [D7 backport] [#2791283]

Follow-up to #2770761: Derivatives should not be created on read-only stream wrappers, for backporting from 8.x to 7.x

Problem/Motivation

If the source image is passed as a stream wrapper, image_style_url() creates the destination on the same stream wrapper. This is fine for public:// and private:// and even for any writable custom stream wrapper. But when it comes to build a derivative from a read-only stream wrapper, Drupal cannot write there because it's simply... READONLY.

This is somehow related to #987846: Switching file storage to anything other than public/private breaks image styles but is not the same in the sense that D8 allows creation of derivatives from sources passed as stream wrappers but will fail to create the files on read-only wrappers.