Rasterisation of layers in vector output

When saving a scatter plot as SVG or PDF output, every point is described and thus file size increases with every point.
In cases of severe overplotting or large datasets, it might be more space efficient to rasterise such layers and include the rasterised image in the output.
I don't think this should happen automatically, but might be an interesting opt-in for layers.
Aside from that, rasterisation is also a basis for various raster effects like blurs, drop shadows, glows, distortions and whatnot. The latter use case is perhaps a bit more frivolous, but applies to raster output as well.

Start by reviewing the rendering and SVG/PDF output entry points for scatter layers. Define the scope of an opt-in layer-level rasterisation feature and identify how rasterized content could be represented in both formats without changing default output. The issue names no files or tests, so those must be located first.