Handle flow trees with block children of inlines.
Handle splitting inlines with interior blocks correctly.
auto merge of #753 : metajack/servo/inline-block, r=eric93

r? @eric93
Drop 'ref' to work around a Rust bug

The bug is mozilla/rust#8675.

Fixes #752.
auto merge of #756 : kmcallister/servo/frametreeiterator, r=metajack

The bug is mozilla/rust#8675.

Fixes #752.
Correct draw order for frame tree layers
Implement compositor layer occlusion; clean up
auto merge of #758 : eschweic/servo/comp-clean-up, r=metajack

Reorder layer tree construction, implement layer occlusion, and general clean up.
Implement collapsing margin