1.x Improve handling of full size images

[gchtr]

Fixes some bugs when working with the full image size.

• Fixes a bug when the full size was used with Image::picture_responsive().
• Introduces Image::is_full_size() to check whether a full size should be returned.
• Introduces Image::auto_full_src() to automatically return the correct URL for a full size based on the size key.
• Introduces a new set_webp() method to enabled/disable WebP for an image with full size key.