Make unrooted_must_root a bit more aggressive.
Basically, instead of trying to check for specific kinds of statements,
just check the types of all local variables.
Also included are some commented-out proposals for some slightly more
aggressive lints which might be useful (but trigger a little too
frequently at the moment).
<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/8073)
<!-- Reviewable:end -->
Integrate Canvas into the DisplayList
Canvas is currently given a layer at the stacking context level.
Instead it's DisplayItem should be given a layer directly. This fixes
painting order issues where canvases are painted on top of other
positioned content that is later in tree order. It always simplifies
the code a bit.
<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/8140)
<!-- Reviewable:end -->
Basically, instead of trying to check for specific kinds of statements,
just check the types of all local variables.
Also included are some commented-out proposals for some slightly more
aggressive lints which might be useful (but trigger a little too
frequently at the moment).
Remove unnecessary rooting from `CSSStyleDeclaration`
This commit will replace calls to `self.owner.root()` with `self.owner` to avoid unnecessary rooting of JS elements objects.
Ref.- Issue: #8126
<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/8137)
<!-- Reviewable:end -->
Remove Rc<T> usage from Range
I initially used this to correctly handle ranges when their respective containers
are mutated, to get weak references of Range objects. I now realise that the weak
references should be handled at a lower level, closer to the JS-managed object.
<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/8091)
<!-- Reviewable:end -->
