"Paint" matches what we call this in the rest of the system. Let's not
confuse things by mixing paint/render/draw all the time. I'm guilty of
this in more places..
Also rename RenderingContext => PaintContext.
CSS defines a very specific paint order. This patch starts steering us
towards respecting that by introducing the PaintPhase enum with values:
- Background
- Border
- Foreground
- Overlay (internal overlays used by inspector)
Basically, to get the right visual result, we have to render the page
multiple times, going one phase at a time.
The more generic virtual variant is renamed to node_name() and now only
Element has tag_name(). This removes a huge amount of String ctor/dtor
churn in selector matching.
To support z-ordering when painting, the layout tree now has a parallel
sparse tree of stacking contexts. The rules for which layout boxes
establish a stacking context are a bit complex, but the intent is to
encapsulate the decision making into establishes_stacking_context().
When we paint, we start from the ICB (LayoutDocument) who always has a
StackingContext and then paint the tree of StackingContexts where each
node has its children sorted by z-index.
This is pretty crude, but gets the basic job done. Note that this does
not yet support hit testing; hit testing is still done using a naive
treewalk from the root.
