SurfacePoint from which to emit.
@param lambda The WavelengthPacket associated with thelight traveling along the path.
@param ru The first random variable (must be in [0, 1]).
@param rv The second random variable (must be in [0, 1]).
@param rj The third random variable (must be in [0, 1]).
@return The ScatteredRay, or null if thismaterial is not emissive.
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