Adding a water island when substrate is true will flatten the landscape

When a landscape weightmap or heightmap update is required, ALandscape::ResolveLayersTexture is called via the ALandscape::UpdateLayersContent()

The problem appears to be that all pixels in the OutMipsData contents are set to the heightmap default values.

1. Create an empty project
2. enable water plugin
3. Check substrate to true in project setting
4. Sculpting in Landspace mode 
5. Add a water island Actor

Test Result : 
landspace is cleared as a plane and sculpting is not possible anymore
(Deleting the waterbrushmanager from the outliner will return the Landscape to normal.)