Stop using Either in render pass node

`Either` is used in render pass node for handling both images and surfaces:

https://github.com/amethyst/rendy/blob/778b47ff793d2fbf4f080f25e930b7402d0d8a01/graph/src/node/render/pass.rs#L30

However, this is simply confusing and does not provide much benefit considering it's an additional dependency as well. We should use a custom enum instead.