Fixes #635

Before, Makie could in some cases pick limits that were too tight for a given categorical axis, for example if categories were added without data:

spec = data((; group = ["B", "C"], value = [1, 2])) *
    mapping(:group, :value) * visual(Scatter)

draw(
    spec,
    scales(
        X = (; categories = ["A", "B", "C", "D"]),
    ),
)

Now, invisible entries are added that keep the limits at least at first-0.5 to last+0.5, which makes sense given that each category always gets a column of width 1 centered around its position. The example above then becomes:

I had actually thought about doing this for a while, not necessarily because of the missing categories edge case, but because widthless plots like scatters look cramped with categorical data, as can be seen above. That normal case now looks like this, which is much more balanced:

Although this PR changes a lot of reference images slightly, I don't consider it breaking, because this is both a bug fix and an aesthetic improvement, where I don't expect anybody to rely on the exact auto-margin they got previously with categorical data.