Cairo renderer implementation of group symbolizer.

This uses the renderer_common header to do most of the heavy
lifting, but otherwise is very similar to the AGG renderer
implementation.
Implement debug symbolizer for Cairo.

Debug symbolizer is useful, and used in a bunch of tests. This
adds debug symbolizer support for Cairo to make it closer to the
capabilities of AGG.
Update tests for debug symbolizer in Cairo.

Adding debug symbolizer for Cairo meant that red boxes appeared in
many of the visual test outputs. This commit replaces them with
the output, after visual inspection. They should now be closer to
the output of the AGG test cases.
Update Cairo reference images for group symbolizer tests.
Merge pull request #7 from MapQuest/mla-group-symbolizer

Cairo implementation of group and debug symbolizers.