Raster tip 🐛

I noticed a small bug where a raster mark throws an error if you use the data-less `raster({ fill: (x, y) => ... })` form and include a tip.

Notebook: https://observablehq.com/d/aff3900cd6feec56

I'm not sure if this is a simple or a complicated fix since it looks like `index` comes up a lot.

