Added line placement test for group symbolizer.

This just tests placement of groups on a line with variable
spacing.
Fix bug in text placement via thunk.

The implementation shared the stored `placements_list` contents,
which meant that each loop through the renderer it was adding an
offset on top of what had gone before.

The simple fix is just to put everything back the way it was
after rendering, but this isn't ideal - it would be better to
offset only the once and keep the data the same.