Detect and ignore normal map G channel inversion setups in materials

[lasa01]

If you want to use DirectX-style normal maps in Blender, you usually add in a green channel inversion setup such as this: Alternatively, an (color) invert node could be used instead of the subtract node.

Since UE already uses DirectX-style normal maps, the exporter should detect and ignore these setups (and not enable the inverted green channel in UE).