Compute pagination after applying scales

[jkrumbiegel]

This PR changes the pagination mechanism. Before, layers were converted to processed layers, and those were then split apart based on their input data. The problem with that approach was that it made each page independent of the others. This can be seen in this MWE:

df = (x = 1:4, y = 1:4, page = [1, 1, 2, 2], color = ["A", "B", "C", "D"])
spec = data(df) *
    mapping(:x, :y, color = :color, layout = :page => nonnumeric) *
    visual(Scatter, markersize = 30)
draw(spec)

When we split this into two pages, we get this on master:

p = paginate(spec, layout = 1)
draw(p, 2)

Both A & C and B & D share the same color in the legend because each page has separately computed categorical scales. (This problem does not appear if all pages have the same categories, but that's not something to generally rely on.)

With this PR, the pages reflect the original colors from the full plot, and the legend also lists all categories on both pages.

In case there are lots of categories that crowd the legend, it might be possible in the future to remove the legend entries for categories that don't appear on a given page, while still keeping the overall colorscheme intact.