add visual test - text data binding

Test for https://github.com/mapnik/mapnik/issues/2433.

However it can fail because of this: https://github.com/mapnik/mapnik/issues/1871

There is also weird behavior of `geometry-transform` in combination with `scale_factor`. See that one circle on the image is completely missing and the other one is deformed to huge ellipse:

![text-data-binding-500-500-1 0-agg](https://cloud.githubusercontent.com/assets/1950911/4456991/335bc6f4-4891-11e4-9617-c1a5e82a9407.png)
![text-data-binding-500-500-2 0-agg](https://cloud.githubusercontent.com/assets/1950911/4456990/334836f2-4891-11e4-901b-9630a9ec0ace.png)

I think geometries should not be affected by `scale_factor`. I'm also not sure if `wrap-width` should be affected by `scale_factor`. At least it's worth separate test case.