Add CMake files as the build system.
Remove VS projects as CMake can generate them.

Also remove old scripts and tag things.
Use external libpng and zlib.
Use external GLUT library.
Fix file extension of triangle.c.
Fix compilation on 64-bit systems.

I'm not crazy about this use of pointers as keys, but it seems to be
okay for the time being. One or two don't make sense, though, so I've
made a comment about that in the code.
Fix library link order.
Remove outdated executables and docs.