Fix preview centering for custom meshes

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

 
             Mesh previewMesh = mesh == null ? s_Meshes[0] : mesh;
 
-            Vector3 previewPosition = previewMesh.bounds.center;
-
-                Matrix4x4.TRS(previewPosition, Quaternion.identity, Vector3.one),
+                mode == PreviewMode.Preview3D ? Matrix4x4.TRS(-previewMesh.bounds.center, Quaternion.identity, Vector3.one) : Matrix4x4.identity,
                 mat,
                 1,
                 m_Camera,