Allow for Custom Layers in ImageLayer and MultiscaleImageLayer

[ilan-gold]

User story
ImageLayer and MultiscaleImageLayer both handle a lot of logic around the loader, tile sizes, doing the loading etc. but it would be nice (i.e for the bitmask use-case in Vitessce) to be able to just pass in a custom layer so you can re-use all the logic for these hairy things while still getting the ability to customize your rendering.

Preferred solution
Add a Layer prop to both ImageLayer and MultiscaleImageLayer.

Possible alternatives
Another possible solution, I think, would be to refactor the shaders to allow for custom rendering, although this would be problematic if you want to attach new uniform variables to the shader, which we do want to do for the bitmask layer.