Replace per-node heap-allocated AtomicRefCounted
AccumulatedVisualContext objects with a single contiguous Vector inside
AccumulatedVisualContextTree. All nodes for a frame are now stored in
one allocation, using type-safe VisualContextIndex instead of RefPtr
pointers.
This reduces allocation churn, improves cache locality, and opens the
door for future snapshotting of visual context state — similar to how
scroll offsets are snapshotted today.
With per-paintable display list command caching now in place, the
separate paint-only properties resolution phase is no longer needed.
Resolution now happens inline during painting and its cost is amortized
since it only runs on cache miss.
Move all property resolution to point of consumption:
- is_visible() and visible_for_hit_testing() compute on the fly
- Filter resolution moved to assign_accumulated_visual_contexts()
- Border radii, outlines computed on access
- Box shadows, backdrop filter resolved inline during painting
- Background resolution moved into paint_background()
- Mask resolution moved to StackingContext::paint()
- Text fragment and SVG stroke properties resolved during painting
