trimesh.m: Overhaul function to support color matrix argument. * scripts/plot/trimesh.m: Add documentation about C matrix argument. Also return handle H for 2-D trimesh (equivalent to triplot). Add code to check for 4th argument being a color matrix. Set FaceColor to white in patch which corresponds to "hidden on". Turn off axis plot box for visual compatibility with Matlab. Add %!error tests for input validation.