MSFT_lods Extension ScreenCoverages Support

[Evgeni_Popov]

See this thread for context on why this part has been removed:

How to implement incremental loading for GLTF Model?

Sorry, @PirateJC mentioned this yesterday. The code that uses screen coverage was reverted because it didn’t work properly. We probably added something to the doc that didn’t get reverted. Right now, there is no glTF loader code that loads the screen coverage value.