Expr v2

Work in progress towards Mapnik 3.x

Includes:

  - Deep refactoring to share more code between renderers
  - Group symbolizer (#2160)
  - Moves many more symbolizer properties to expressions like colors, opacity, stroke-width (work in progress: enumerations not yet supported and text symbolizer also not yet improved)
  - partial @attribute support in expressions (work in progress)
  - various c++11 improvements
  - attempt (that needs testing) to better handle interior polygons in shapefiles (https://github.com/mapnik/mapnik/commit/ecc5acbdb953e172fcc652b55ed19b8b581e2146)
  - C++ version of nik2img
  - Various consistency fixes to copy ctors