Reimplement the Imaginate node and viewport tool

[Keavon]

Blocked on #1270

Following the migration to the unified node graph, the previous Imaginate functionality has changed. We no longer rasterize the area below the Imaginate frame now that compositing is part of the single document graph.

For now, users can use the Imaginate feature by inserting the Imaginate node in the graph inline with the flow of raster data, like so:

The Imaginate tool should be rebuilt to work based on the new graph-based layer compositing system with a draggable frame.

The Imaginate node also needs to be able to rasterize vector data fed into it, since it only supports raster for the moment. Deciding precisely how it handles its frame bounds in different circumstances is still up for design discussion.