Position layers using their absolute position

Layers are currently all children of the root layer, so instead of
using coordinates relative to the parent flow we should use coordinates
relative to the page.

Fixes #2061.
auto merge of #3617 : mrobinson/servo/layer-positioning, r=pcwalton

Layers are currently all children of the root layer, so instead of
using coordinates relative to the parent flow we should use coordinates
relative to the page.

Fixes #2061.