Visual comparisons instead of geojson text comparisons

[springmeyer]

We need to add support for visual comparisons of changes in geometry outputs. Minor changes in floating point differences currently trigger the need for massive updates to the test fixtures like mapnik/geometry-test-data@65abcf0 which is dangerous because:

• The diff is way to big for github to show
• Meaningful changes can too easily go unnoticed.

A visual approach (rendered png) would make it viable to review all the changes efficiently rather than just hope for the best.

[pnorman]

pretty printing the json might also help, it would mean that when one value changes you can get a more meaningful diff.