undo material changes (#3503)

Project/Assets/ML-Agents/Examples/SharedAssets/Materials/White.mat

 Material:
   serializedVersion: 6
   m_ObjectHideFlags: 0
-  m_CorrespondingSourceObject: {fileID: 0}
-  m_PrefabInstance: {fileID: 0}
-  m_PrefabAsset: {fileID: 0}
+  m_PrefabParentObject: {fileID: 0}
+  m_PrefabInternal: {fileID: 0}
   m_Name: White
   m_Shader: {fileID: 46, guid: 0000000000000000f000000000000000, type: 0}
   m_ShaderKeywords: _GLOSSYREFLECTIONS_OFF _SPECULARHIGHLIGHTS_OFF

Project/Assets/ML-Agents/Examples/WallJump/Materials/TransparentWall.mat

 Material:
   serializedVersion: 6
   m_ObjectHideFlags: 0
-  m_CorrespondingSourceObject: {fileID: 0}
-  m_PrefabInstance: {fileID: 0}
-  m_PrefabAsset: {fileID: 0}
+  m_PrefabParentObject: {fileID: 0}
+  m_PrefabInternal: {fileID: 0}
   m_Name: TransparentWall
   m_Shader: {fileID: 46, guid: 0000000000000000f000000000000000, type: 0}
   m_ShaderKeywords: _ALPHABLEND_ON
     - _UVSec: 0
     - _ZWrite: 0
     m_Colors:
-    - _Color: {r: 0.043137256, g: 0.0627451, b: 0.5764706, a: 1}
+    - _Color: {r: 0.56228375, g: 0.76044035, b: 0.9558824, a: 0.603}
     - _EmissionColor: {r: 0, g: 0, b: 0, a: 1}

Project/Assets/ML-Agents/Examples/WallJump/Materials/spawnVolumeMaterial.mat

 Material:
   serializedVersion: 6
   m_ObjectHideFlags: 0
-  m_CorrespondingSourceObject: {fileID: 0}
-  m_PrefabInstance: {fileID: 0}
-  m_PrefabAsset: {fileID: 0}
+  m_PrefabParentObject: {fileID: 0}
+  m_PrefabInternal: {fileID: 0}
   m_Name: spawnVolumeMaterial
   m_Shader: {fileID: 46, guid: 0000000000000000f000000000000000, type: 0}
   m_ShaderKeywords: _ALPHABLEND_ON
     - _UVSec: 0
     - _ZWrite: 0
     m_Colors:
-    - _Color: {r: 0.09411766, g: 0.098039225, b: 0.7568628, a: 1}
+    - _Color: {r: 0, g: 0.83448315, b: 1, a: 0.303}
     - _EmissionColor: {r: 0, g: 0, b: 0, a: 1}