Add --with-png configure option
Use libpng reader
Merge branch 'feature-libpng-integration' into release-1.3

Conflicts:
	configure
	configure.ac
	converters/Makefile.am
	converters/Makefile.in
	converters/loader.c
Fix build error
Merge branch 'feature-libpng-integration' into release-1.3

Conflicts:
	converters/loader.c