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.
Refactor compositor to use messages rather than callbacks.