add support for multiple layers

[kmader]

cornerstone has native support for layers and transparency and it would be useful to output segmentations and heatmaps and show multi-modal images like PET-CT

relevant JS is https://github.com/cornerstonejs/cornerstone/blob/803dd7335e16de3cc398a3d42eda2552dd47cb1b/example/layers/index.html#L95-L152