use non-zero filling rule when rasterising lines

src/agg/process_line_symbolizer.cpp

@@ -164,6 +164,7 @@ void agg_renderer<T>::process(line_symbolizer const& sym,
         renderer_type ren(renb);
         ren.color(agg::rgba8_pre(r, g, b, int(a * stroke_.get_opacity())));
         agg::scanline_u8 sl;
+        ras_ptr->filling_rule(agg::fill_non_zero);
         agg::render_scanlines(*ras_ptr, sl, ren);
     }
 }

src/cairo_renderer.cpp

@@ -497,6 +497,7 @@ void cairo_renderer_base::process(line_symbolizer const& sym,
         }
     }
     // stroke
+    context_.set_fill_rule(CAIRO_FILL_RULE_WINDING);
     context_.stroke();
 }
 

src/grid/process_line_symbolizer.cpp

@@ -106,6 +106,7 @@ void grid_renderer<T>::process(line_symbolizer const& sym,
 
     // render id
     ren.color(color_type(feature.id()));
+    ras_ptr->filling_rule(agg::fill_non_zero);
     agg::render_scanlines(*ras_ptr, sl, ren);
 
     // add feature properties to grid cache