Grid markers fix for issue #2500

This is a fix for #2500. A new test case is added that isolates the issue being fixed. This also affects several of the existing grid test cases, however. These are changed to better match the agg and cairo output, which is most easily seen in functional-expressions-length test case.

I fixed this problem in the process of adding markers support in group symbolizer, but I wanted to separate this into its own pull request to avoid mixing this test changes in with that.