Removing layer replacement.
Asserting `ImageInterface#layers()` returns object that's instance of `LayersInterface`.
Asserting created or opened images will implement `LayeredImageInterface`.
Asserting `ImageInterface#layers()` will implement `ArrayAccess`.
Asserting created and opened images will implement `ArrayAccess`.
Testing layer replacement.
Layer interface check superfluous.
Check for `ArrayAccess` not needed.
Adding accidentally removed newline.
Asserting creating an image without a specific background color results in a white background.
`Imagine` should be able to create an image without any layers.
`LayeredImageInterface` should be `Countable`.
`ImageInterface#layers()` should return itself.
Adding tests that create an animated GIF from scratch.
Layers should implement `ArrayAccess` and `Countable`.
Addings test for deleting layers.