Steps to reproduce :

- create a terrain
- go to Paint the terrain texture
- assign a texture
- assign a second texture
- try to paint using the second texture and notice that it doesn't work

Exception: Float array size wrong (layers should be 2)
UnityEngine.TerrainData.SetAlphamaps (Int32 x, Int32 y, System.Single[,,] map) (at C:/buildslave/unity/build/artifacts/generated/common/modules/Terrain/TerrainDataBindings.gen.cs:656)
UnityEditor.SplatPainter.Paint (Single xCenterNormalized, Single yCenterNormalized, Int32 splatIndex) (at C:/buildslave/unity/build/Editor/Mono/TerrainEditor/TerrainInspector.cs:121)
UnityEditor.TerrainInspector.OnSceneGUICallback (UnityEditor.SceneView sceneView) (at C:/buildslave/unity/build/Editor/Mono/TerrainEditor/TerrainInspector.cs:1892)
UnityEditor.SceneView.CallOnSceneGUI () (at C:/buildslave/unity/build/Editor/Mono/SceneView/SceneView.cs:2397)
UnityEditor.SceneView.HandleSelectionAndOnSceneGUI () (at C:/buildslave/unity/build/Editor/Mono/SceneView/SceneView.cs:1715)
UnityEditor.SceneView.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/SceneView/SceneView.cs:1585)
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)

*Note :
Regression from 2017a3.