Fix crash on window resize
auto merge of #587 : eschweic/servo/master, r=metajack

This fixes a bug that would sometimes trigger a fail! when the window was resized.
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.