Add a trait for FlowContext.
Abstract the process of attaching boxes to flows. Remove parent-child box pointer setup.
Remove RenderBox tree and traversals; refactor box builder to pass BuilderContext around; add and thread through a stub BoxConsumer implementation.
Silence noncopyable typaram warnings
RCU -> COW
Convert doc attributes to comments
Move an allow(non_implicitly_copyable_typarams) attribute to a smaller scope
Revert the addition of accept_new_box() implementations for each flow type.
Build inline flow box list using BoxConsumer. Fix failure cases in
FlowContext variant helper methods to only print flow/box id, so they
don't run out of stack when reflectively printing cycles.
Replace remaning doc attrs with comments
Remove #[legacy_modes]
Remove legacy_modes harder
Update for Rust library changes
Remove exports in osmain
Fix parser crash. Fix competing uses of JSContext's private slot.
Add a LayerBuffer abstraction instead of sending around DrawTargets directly.

A step on the way toward window resizing.
Speed up JS bindings by using Rust jsval manipulation inlining.
Fix build error
Stub RenderLayers.

This is a step on the way toward window resizing.
gfx: Send a RenderLayer instead of a display list from layout to GFX.

This is a step on the way to window resizing.