SciTools/cartopy — pull request #452 by rhattersley (2014-06-27 10:56:04+00:00)
Title: Allow for very minor source/rendering differences.

An example diff image that the new tolerance just about lets pass:
![result-web_tiles-failed-diff](https://cloud.githubusercontent.com/assets/917914/3410602/8e02949a-fde9-11e3-9df8-c8045e56e480.png)