Update the filepath of an image when it already exists in the main storage. This is useful when `relative_path` property is changed and we attempt to open the same image file again. Also remove `is_relative_path` parameter. Not required anymore because `range->filepath` is correctly set to absolute/relative. This is also passed in `BKE_image_load_exists_ex` to set path to non-exiting image. Pull Request: https://projects.blender.org/blender/blender/pulls/109815