Previously, the column count was always incremented by 1. This led to a
mismatch with `compute_outer_content_sizes()`, which did use the `span`
attribute to advance the column index. This mismatch caused an
out-of-bounds access when the column index was greater than the
expected number of columns.
In LayoutState, used_values_per_layout_node should not be modified in
order to determine inline nodes' dimensions - all the required values
should already be in there. In 2585f2da0d
we did accidentally create new values, causing the code further down to
try and get a PaintableBox from an anonymous container and crashing.
Fixes#6015.
This fixes the very, _very_ slow loading of https://yzy-sply.com. The
`apply_style()` method also calls into this method recursively, so we
just need to call it once instead of once per node in the continuation
chain.
