Vector patterns with Cairo

[talaj]

Cairo backend is rasterizing SVG patterns prior to rendering. This leads to poor result when rendering to PDF or SVG. This pull request adds ability to render SVG patterns as vectors.

The implementation is using Cairo recording surface. An SVG is rendered to the recording surface and this surface is then used as a pattern.

[codecov]

Codecov Report

Merging #4009 into master will decrease coverage by 0.17%.
The diff coverage is 20.4%.

@@            Coverage Diff             @@
##           master    #4009      +/-   ##
==========================================
- Coverage   71.69%   71.51%   -0.18%     
==========================================
  Files         441      441              
  Lines       23391    23029     -362     
==========================================
- Hits        16770    16470     -300     
+ Misses       6621     6559      -62

Impacted Files	Coverage Δ
include/mapnik/cairo/cairo_context.hpp	100% <100%> (+100%)	⬆️
include/mapnik/cairo/render_polygon_pattern.hpp	42.85% <15.21%> (-28.58%)	⬇️
include/mapnik/symbolizer_base.hpp	0% <0%> (-100%)	⬇️
include/mapnik/marker_cache.hpp	0% <0%> (-100%)	⬇️
include/mapnik/boolean.hpp	0% <0%> (-100%)	⬇️
include/mapnik/featureset.hpp	0% <0%> (-100%)	⬇️
include/mapnik/raster_colorizer.hpp	0% <0%> (-85.72%)	⬇️
include/mapnik/util/singleton.hpp	0% <0%> (-85%)	⬇️
plugins/input/csv/csv_utils.hpp	18.18% <0%> (-77.28%)	⬇️
include/mapnik/params.hpp	0% <0%> (-66.67%)	⬇️
... and 31 more

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[artemp]

@talaj - great feature! Is this PR ready for merging ?

[talaj]

@artemp Yes.

[artemp]

@talaj - could you help fixing data-visual . I'm locally getting pattern alignment failures. I wonder if data-visual needs rebasing ?

[talaj]

@artemp - Sure, I will take a look.

[artemp]

👍

[talaj]

Rebased.