Create a group_symbolizer_helper for group placments, and extract some code from
text_symbolizer_helper into a base class to share with group_symbolizer_helper.
Also, move tolerance_iterator into its own header file. Use helper in
process_group_symbolizer to find placement positions.
This includes XML parsing of group symbolizer and related objects and
process_group_symbolizer method in the AGG renderer. This also includes
code to collect group symbolizer indexed columns, create sub features,
and match them to group rules.
This forces offset lines to be aligned to the closest point to
the anchor point on the original line, meaning that they are
aligned where the offset line and original run parallel, or nearly
so.
- Implementation by @herm for GSOC 2012 (http://mapnik.org/news/2012/10/06/gsoc2012-status9/)
- C++11 port, improvements, optimizations by @artemp
- Testing and integration with master by @springmeyer
- Thank you to all the support from @behdad along the way
- Thanks for help testing @toton6868, @stephankn, @nirvn, @mfrasca, @simonsonc and many others
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