add support for halo-rasterizer for shields - closes #2226

Conflicts:
	src/load_map.cpp
	tests/visual_tests/images/shield-on-line-spacing-eq-width-600-400-1.0-agg-reference.png
	tests/visual_tests/images/shield-on-line-spacing-eq-width-600-400-2.0-agg-reference.png

src/load_map.cpp

@@ -1327,6 +1327,8 @@ void map_parser::parse_shield_symbolizer(rule & rule, xml_node const& sym)
         ensure_exists(file);
         put(shield_symbol, keys::file , parse_path(file, sym.get_tree().path_expr_grammar));
         parse_symbolizer_base(shield_symbol, sym);
+        optional<halo_rasterizer_e> halo_rasterizer_ = sym.get_opt_attr<halo_rasterizer_e>("halo-rasterizer");
+        if (halo_rasterizer_) put(shield_symbol, keys::halo_rasterizer, halo_rasterizer_enum(*halo_rasterizer_));
         rule.append(std::move(shield_symbol));
     }
     catch (config_error const& ex)

tests/visual_tests/styles/shield-on-line-spacing-eq-width.xml

@@ -69,6 +69,7 @@
               face-name="DejaVu Sans Book"
               halo-fill="white"
               halo-radius="2"
+              halo-rasterizer="fast"
               spacing="15">
             'hi'
             </ShieldSymbolizer>