Account for RenderLayer position when optimizing display list

The page_rect passed to DisplayListOptimizer is relative to the
RenderLayer origin, but the display list components are relative to the
page origin. Before passing the page rect to the display list, we
translate it by the RenderLayer position.
auto merge of #3567 : mrobinson/servo/optimizer, r=zwarich

The page_rect passed to DisplayListOptimizer is relative to the
RenderLayer origin, but the display list components are relative to the
page origin. Before passing the page rect to the display list, we
translate it by the RenderLayer position.