Refactor display lists to use an explicit traversal
auto merge of #589 : eric93/servo/diplaylist-refactor, r=pcwalton
Add rust-png submodule.
Enable `-o` option for output_file.
Bump rust-azure pointer.
Clean up check targets in Makefiles.
Write out PNG file after compositing based on `-o` option.
Update README with new deps.
Update rust-png to support libpng 1.2.
auto merge of #590 : metajack/servo/png-output, r=metajack,me

This enables the `-o FILE` command line option to render the window contents to a PNG after rendering and exit Servo.
Update remove_tile, add automatic tile removal, fix tests for quadtree
auto merge of #591 : eschweic/servo/del-tiles, r=pcwalton

The quadtree is now initialized with a memory limit, and tiles are deleted automatically if that limit is exceeded. The memory limit can also be set to None to prevent this behavior.
Refactor compositor to use messages rather than callbacks.
Added mozharness config files
Fix regression on clicking links
auto merge of #597 : eric93/servo/diplaylist-refactor, r=jdm
Bump protoArray size for WindowProxy

Fixes #596
auto merge of #599 : kmcallister/servo/proto-array, r=jdm

Fixes #596

@jdm r?
Rewrite reftest harness and add basic example reftest.
auto merge of #594 : jsyeo/servo/add-MH-configs, r=metajack

For https://bugzilla.mozilla.org/show_bug.cgi?id=891870.