[unity] Fixed SkeletonMecanim MixModes being incorrectly initialized (`AlwaysMix` vs `MixNext`) when `Custom MixMode` is enabled and the Component was just created/reset or new Mecanim layers were added.

spine-unity/Assets/Spine/Runtime/spine-unity/Components/SkeletonMecanim.cs

@@ -278,7 +278,7 @@ namespace Spine.Unity {
 						bool isAdditiveLayer = false;
 						if (layer < layerBlendModes.Length)
 							isAdditiveLayer = layerBlendModes[layer] == MixBlend.Add;
-						layerMixModes[layer] = isAdditiveLayer ? MixMode.MixNext : MixMode.AlwaysMix;
+						layerMixModes[layer] = isAdditiveLayer ? MixMode.AlwaysMix : MixMode.MixNext;
 					}
 				}