examples: add ported two-cubes example (#228)

build.zig

@@ -28,6 +28,7 @@ pub fn build(b: *std.build.Builder) void {
         .{ .name = "triangle" },
         .{ .name = "boids" },
         .{ .name = "rotating-cube", .packages = &[_]Pkg{Packages.zmath} },
+        .{ .name = "two-cubes", .packages = &[_]Pkg{Packages.zmath} },
     }) |example| {
         const example_exe = b.addExecutable("example-" ++ example.name, "examples/" ++ example.name ++ "/main.zig");
         example_exe.setTarget(target);