Thunder's lightning is always visible with material system enabled

[illwieckz]

Something I found in the last minutes before building 0.56, but since it only affected the Material System which is disabled by default, and that the bug is not that big (we had it in another form for years) I didn't want to delay the release because of that.

Now writing it to not forget.

Thunder's lightning is always visible with material system enabled.

[slipher]

This is presumably because shader replacement is not implemented.