src/img.c,src/img.h: Update to the version from 1.1.13 so that we
can read version 6 .3d files written by newer 1.1.x releases.
src/dump3d.c,src/extend.c,src/netskel.c: Set img_output_version to 3
before writing out a .3d file for compatibility with older 1.0.x
releases. We don't use any of the newer features, so there's no
benefit to using a newer format version.
