grid.  So now you can convert from image to hog instead of just hog to image.  Also added some
related unit tests.

--HG--
rename : tools/imglab/src/convert_pascal_voc.h => tools/imglab/src/convert_pascal_xml.h

--HG--
rename : tools/imglab/src/convert_pascal_voc.cpp => tools/imglab/src/convert_pascal_xml.cpp

imglab annotation format.

I did this because png.h will cause the compile to fail if the user #includes setjmp.h
along with png.h.

Also added interface functions to programmatically observe what is happening to the
overlays.