`Imagine\Gmagick\Layers` didn't import `ImageInterface`.
Merge branch 'delays-replace-layers' of github.com:reprovinci/Imagine into delays-replace-layers
Loaded and created images should implement `LayeredImageInterface`.
This interface should extend `ArrayAccess` and `Countable`.
Created images should have a white background.
Testing `LayeredImageInterface#layers()` returns itself.
If this is true, all other tests should also cover `LayeredImageInterface`.
Modifying layer replacement to work with `ArrayAccess`.
Asserting layer has been replaced before save.
Testing addition of layers.
Also testing "saving" in layer addition test.
Testing layer removal.
Testing layer counting.