Make the Rasterize node resolution-aware

[Keavon]

The current Rasterize node is a major footgun because its bounds have to be set manually, which multiple users have complained about feeling broken because they didn't realize a width, height, and X/Y offset had to be chosen to make use of the node.

We want to instead use the minimum of the content bounds and the viewport bounds (the context's footprint), and with a resolution assigned by the viewport resolution from the context's footprint. This lets us eliminate the bounds parameter of the Rasterize node so that node works without needing any parameters.

This makes the "Blend" node no longer necessary, because that is simply equivalent to a stack of two raster layers that feed into a Rasterize node. So that node can also be removed.

To allow the user to choose rendering bounds, we also need a separate Crop node. A user would apply a Rasterize node followed by a Crop node to achieve the same outcome. Perhaps also a Resample node to change the rendering resolution for upsampling/downsampling.