Raster reorg

[mkeeter]

This PR is a big reorganiaztion of fidget::raster; this is a step towards merging the WebGPU backend, which wants a similar module organization for the WebGPU backend.

• render2d and render3d modules are renamed to pixel and voxel, respectively
• Types from fidget::raster::config are moved to either pixel or voxel modules
• ImageRenderConfig and VoxelRenderConfig are both renamed to RenderConfig (in different namespaces)
• Similarly, each namespace contains an Image and RenderSize type definition. voxel::Image replaces GeometryBuffer; pixel::Image is equivalent to fidget::raster::Image<DistancePixel>.
• Renamed DistancePixel to RawDistancePixel; added an unpacked enum DistancePixel (instead of using Result<f32, PixelFill> as a weird Either type
• The tile sizes member of RenderConfig is now optional and will fall back to the evaluator's tile size if not populated
  • This adds a F: RenderHints trait bound for many functions in fidget::raster