boxplot should materialize its channels?

[Fil]

if we added, say:

  x = valueof(data, x);
  y = valueof(data, y);

then a boxplot of random numbers would be correct. Currently this shows inconsistencies between the different submarks:

export async function boxplotRandom() {
  return Plot.boxX({length: 20}, {x: d3.randomLcg(42), tip: true}).plot({
    style: "overflow: visible;"
  });
}

However it's possible that we would be better off using creator from the pointer-highlight branch to create the box plot as a single mark instead?

[mbostock]

We can do this using an initializer and context.getMarkState like the tip option does.

[Fil]

This would also allow to optimize the computations, I think?

[Fil]

Related to #1843 — we're computing all these values repeatedly, but should just do it once overall.