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 https://github.com/mapnik/geometry-test-data/commit/65abcf0600d25aefebfe77c62261d4c1d7ac4d31 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.