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