Backend support for layers and vertical compositing stacks

[Keavon]

Layers are a special type of node that:
- [ ] Have a lock icon (this could come after the MVP though)
- [ ] Hold a linear chain of input nodes (this could come after the MVP though)

• Thumbnails for the layer node #1210
• Accept a primary input
• Accept a vertical compositing (monadic) input
• Apply the monadic compositing operation and provide vertical compositing output
• Provide a layout for configuring certain settings on a per-layer basis for vertical compositing (e.g. blend mode and opacity)

This is largely a form of (visual) syntax sugar over the existing node graph implementation. The internal graph doesn't have a concept of horizontal or vertical, or even positioning, so this is just a regular node that we store in a special way to provide the frontend with is unique UI. It requires an updated API for describing the changes to the UI for representing layers vs. regular nodes.

Vertical compositing works like so:

• Each data type used in the node graph can define a vertical compositing monadic operation (a trait)
• The bottom layer in a vertical compositing stack can be either the wrap/constructor for the compositing monad, or it can use a default value and the first layer becomes the first chained application of the operation
• The top output unwraps the monadic compositing value and sends it out as horizontal output data

We need vertical compositing operations for both raster and vector, initially.