gdk-pixbuf: add an option so that loader errors are fatal
If an environment variable is specified set the return value from main() to non-zero if the loader had errors (missing libraries, generally).
The environment variable is used in yocto at build time to catch incorrectly configured loaders: GDK_PIXBUF_FATAL_LOADER=1 ${STAGING_LIBDIR_NATIVE}/gdk-pixbuf-2.0/gdk-pixbuf-query-loaders --update-cache || exit 1