Add unsafe_layout_node(), unsafe_paintable(), and unsafe_paintable_box()
accessors that skip layout-staleness verification. These are for use in
contexts where accessing layout/paintable data is legitimate despite
layout not being up to date: tree construction, style recalculation,
painting, animation interpolation, DOM mutation, and invalidation
propagation.
Also add wrapper APIs on Node to centralize common patterns:
- set_needs_display() wraps if (unsafe_paintable()) ...set_needs_display
- set_needs_paint_only_properties_update() wraps similar
- set_needs_layout_update() wraps if (unsafe_layout_node()) ...
And add Document::layout_is_up_to_date() which checks whether layout
tree update flags are all clear.
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.
Some of this is rearranged for clarity, but it's mostly the exact same
code. Steps 3, 10, 11, and 15 are new, but don't have any effect until
we implement downward-growing cells.
When a table row (or its group) is set to collapse, the row takes up no
vertical space in the layout.
We have to account for this in multiple places, so I've cached whether a
row is collapsed in the TableGrid::Row.
Before this change, we were always processing all row groups first,
and then all rows. This led to incorrect table layouts when rows and row
groups were encountered in anything but that order.
