Shading method clone() method

[richardolsson]

Now that it is explicitly impossible to reuse methods across materials (it would always yield undefined results) it would be of great help to have a clone() method implemented on all shading methods. It could internally use the copyFrom() method which is already on ShadingMethodBase, however not yet implemented on all the concrete shading methods.

[richardolsson]

Shading methods will be shareable again soon, but there should still be clone methods on them. Implement these ASAP (but not a showstopper.)