Updated grid renderer for `expr-v2` branch.

All the grid visual tests pass.

One small issue is that in the grid line pattern symbolizer it
constructs a 'fake' line symbolizer to re-use code (as the grid
renderer cares nothing for patterns), but doesn't pass the
offset parameter to it. The tests currently require this
behaviour, but they should probably be changed as it seems
incorrect.
Merge pull request #2099 from MapQuest/mla-expr-v2-grid

Update grid renderer for `expr-v2` branch