Center preview mesh

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/MaterialGraphPreviewGenerator.cs

 
             Mesh previewMesh = mesh == null ? s_Meshes[0] : mesh;
 
+            Vector3 previewPosition = -previewMesh.bounds.center;
+
-                Matrix4x4.TRS(Vector3.zero, Quaternion.identity, Vector3.one),
+                Matrix4x4.TRS(previewPosition, Quaternion.identity, Vector3.one),
                 mat,
                 1,
                 m_Camera,