Cairo implementation of group and debug symbolizers.

These two symbolizers now work with Cairo, and pass the tests. Previously, the group symbolizer tests had been copied from AGG, but there are some small (generally sub-pixel) differences between the two which meant that the Cairo tests had to be updated from the generated images.

Adding debug symbolizer support also changed the test output, and has been updated. The new test output is much closer to the AGG reference output, due to the inclusion of the debug boxes.