Landscape cooked heightmap and Physical Material mismatch

Several users have reported crashes where the Heightfeild loaded from the DDC and physical material list do not match.

Suggest adding the number of physical materials to part of the cooked heightmap data DDC key which will make it a lot more robust.

First crash:

1. launch the editor from a standalone install (without shared DDC)
2. create a landscape
3. create material with 2 layers and assign to landscape
4. create 2 weight-blend layerinfo objects for the layers inside the landscape tool
5. paint both layers on the landscape
6. save all.
7. launch the editor from another standalone install (without DDC).
7a You can use the same install as (1) if you delete the project's DerivedDataCache folder.
7b OR copy the contents from steps 1-6 across to the second computer or second install.
8. open both landscape layer info objects and assign physical materials to them
9. load the landscape level
10. load another level (do not save the landscape level)
11. load the landscape level again
12. Play in editor.
13. Shoot areas of the landscape where both layers are painted.
14. One area will cause a crash in ConvertQueryImpactHit

Second crash:

1. Create a landscape
2. Create material with 2 layers and assign to landscape
3. Create 2 weight-blend layerinfo objects for the layers inside the landscape tool
4. Create 2 physical materials and assign them to the layerinfo objects
5. Paint landscape layers.
6. Save everything except one of the PhysicalMaterials
7. Exit the editor, restart and reload the level
8. Crash on load in CookCollisionData

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