Merge pull request #1201 from Unity-Technologies/fix/2018.1_HDMaterialconverter

Fix/2018.1 hd materialconverter

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Editor/Material/Lit/StandardsToHDLitMaterialUpgrader.cs

 
                 TextureCombiner maskMapCombiner = new TextureCombiner(
                     metallicMap, 0,                                                     // R: Metallic from red
-                    occlusionMap, 0,                                                    // G: Occlusion from red
-                    detailMaskMap, 0,                                                   // B: Detail Mask from red
+                    occlusionMap, 1,                                                    // G: Occlusion from green
+                    detailMaskMap, 3,                                                   // B: Detail Mask from alpha
                     smoothnessMap, (srcMaterial.shader.name == Standard_Rough)?-4:3     // A: Smoothness Texture from inverse greyscale for roughness setup, or alpha
                 );