Replace the use of Assert.IsNull/IsNotNull() with NavMeshData arguments because the methods don't check correctly the null of UnityEngine.Object.

Move the NavMeshData asset management code from NavMeshSurfaceEditor to NavMeshDataAssetManager in order to separate responsibilities.

Moved NavMeshSurfaceEditor.cs to NavMeshDataAssetManager.cs in order to turn it into a copy of the file.

When baking a new NavMesh surface inside a prefab do not delete the old asset until the prefab has been saved, because it will be needed again if the changes are discarded.

Added test to check that the variant of a prefab containing a NavMeshSurface does not delete the asset of the original prefab.

This way the asset is saved in the same folder as the prefab.

Updated test to check that baking a NavMeshSurface inside a prefab does not delete the original NavMeshData until the prefab is saved, after which the original asset should definitely be deleted.

Use the public PrefabStageUtility.GetCurrentPrefabStage() instead of the internal StageNavigationManager.

Remove unneeded marking of dirty scene in a PrefabTest because it is already handled by the NavMeshSurfaceEditor code.

API update: prefabInstanceRoot got renamed to prefabContentsRoot and IsPartOfPrefab got renamed to IsPartOfPrefabContents.

Create a temporary scene for running the tests, in order to better contain the created game objects and assets.

Change prefab tests to use the NavMeshSurfaceEditor methods for baking and clearing the NavMesh data.

Added test to check that instance's NavMeshData .asset file gets deleted when the instance reverts back to prefab structure.

Added test to check that the prefab's NavMeshData .asset file is deleted when an instance pushes a new asset to the prefab.

Added tests for checking that a prefab baking a new NavMeshData.asset file replaces the old one only when the prefab gets saved.
When the prefab's NavMeshData changes get discarded the .asset file should be deleted.

Revert "NavMeshSurfaces are now responsible for displaying their debug representation in the Editor."

This reverts commit 3f4bad4e.

Random seed is local to the tile - removed member.
Pool size overflow: fixed detection and added console warning.