…compositing modes - refs #1206
The png library uses setjmp/longjmp to throw exceptions when reading. If this is not set up, the png library calls abort(). This change handles the errors and throws a C++ exception instead. This issue was found by testing images from pngsuite at http://www.schaik.com/pngsuite/. These images are included and a unit test was added to test both images that should be successful and images that should throw an exception.
(cherry picked from commit 173c402) Conflicts: include/mapnik/vertex_converters.hpp src/agg/process_markers_symbolizer.cpp src/agg/process_point_symbolizer.cpp src/agg/process_polygon_pattern_symbolizer.cpp src/load_map.cpp
…ion into separate namespace (cherry picked from commit 653bca6)