Make box_ not Option in block flows.(fixes #2012)
Destroy LayerBuffers before dropping them

This should fix a potential pixmap leak and task failure when the LayerBuffer
is dropped (#1187).
inline pseudo element
Clear tiles before dropping CompositorLayer

This fixes a pixmap leak and task failure (#2069).
Rename before_remove_attr and after_set_attr to allow reusing those names in the next commit.
Remove unused create_layer_group_for_pipeline
Don't reuse an old pipeline for a new root layer

When navigating to a new URL, we need to make sure compositor messages go to
the new page's pipeline, not the previous one.
Fix HTMLElementDerived::is_htmlelement to take non-HTML elements into account.
Add support for trait-based virtual methods on Nodes, and use it for before_remove_attr and after_set_attr.
Integrate Element's after_set/remove behaviour into the virtual method hierarchy.
Add overridable tree binding/unbinding behaviour.
Remove the IElement trait.
Use ToJSValConvertible for all conversions to JSVal.
Remove getWrapTemplateForType and simplify wrapForType.
auto merge of #2068 : mbrubeck/servo/1187-buffer-destroy, r=larsbergstrom

This fixes a potential pixmap leak and task failure when the LayerBuffer is dropped (#1187).

I wasn't able to reproduce the failure using the steps from #1187, but based on the stack trace I think this is the correct fix.  Alternately, would it make sense to `impl Drop for LayerBuffer`?
Use clear_all_tiles when deleting a sublayer
auto merge of #2020 : lpy/servo/issue2012, r=larsbergstrom

see #2012
auto merge of #2084 : mbrubeck/servo/navigate-new-pipeline-fix, r=larsbergstrom

After clicking a link to load a new page, the new page content is never painted.  This is because the compositor is still sending render messages to old frame tree's pipeline, not the new one.

This patch fixes this by ensuring that the compositor's new root layer is attached to the correct pipeline. (Note: You will also need to apply #2080 if you want to test this without crashing.)
auto merge of #2086 : Ms2ger/servo/htmlelement-cast, r=jdm
auto merge of #2088 : Ms2ger/servo/global-ToJSValConvertible, r=jdm