Fix T53707: Error on export FBX object [PointLighs].

Inverse Coefficients decay method was added later, never made it to FBX
exporter.

Note that this is very limited approxiamtion/simplification here, we
cannot really export this correctly.

To be backported to 2.79a.

Conflicts:
	io_scene_fbx/__init__.py

io_scene_fbx/fbx_utils.py

@@ -94,6 +94,7 @@ FBX_LIGHT_DECAY_TYPES = {
     'CONSTANT': 0,                   # None.
     'INVERSE_LINEAR': 1,             # Linear.
     'INVERSE_SQUARE': 2,             # Quadratic.
+    'INVERSE_COEFFICIENTS': 2,       # Quadratic...
     'CUSTOM_CURVE': 2,               # Quadratic.
     'LINEAR_QUADRATIC_WEIGHTED': 2,  # Quadratic.
 }