Some streams do not support "stat()", but will still allow reading and
writing.

This separates the logic for writing to streams from "_save" into
_write_image.  It forces the call through "make_image" to match the
existing pattern.  "make_image" itself is overidden to unconditionally
invoke its callback.

In terms of compatibility, in the unlikely case that anyone is relying
on WP_Image_Editor_Imagick::make_image, it will work just the same for
plain files.  Stream support is new behaviour, so a caller will just
have to ensure their callback handles URLs appropriately.

Overriding make_image will likely make it incompatible with existing
callers.  Don't use it.