Add a noimpact surface flag similar to func_nogrenades

[Mac-tf2]

Having a surface flag that can fizzle rockets similar to how skybox brushes or func_nogrenade surfaces currently do would be helpful when anticheating maps in weapon modes. Instead of having to place func_nogrenades on every wall, you would add "%compilenoimpact" to the VMT for surfaces that you dont want rockets to explode on.

This behaviour also exists in quake3 as a shader parameter, so this would be needed for bspconvert to be able to port those maps.