gfx: Micro-optimize `DList` and `StackingContext` a bit.

I'm sad to say that this improved performance significantly. A lot of
this win is due to the Rust compiler not being smart about not zeroing
objects out if it doesn't need to.
auto merge of #3890 : pcwalton/servo/microoptimize-dlist, r=glennw

I'm sad to say that this improved performance significantly. A lot of
this win is due to the Rust compiler not being smart about not zeroing
objects out if it doesn't need to.

r? @glennw