VRTheWorld terrain provider broken

[lilleyse]

VRTheWorld terrain provider is broken in master but not in 1.55. Git bisect points to b772d52. It probably has something to do with flipY getting set incorrectly for heightmap terrain.

@OmarShehata could you check this out?

[OmarShehata]

Can confirm it's the image bitmap. If you add:

Cesium.Resource.supportsImageBitmapOptions = function(){
    return Cesium.when.resolve(false);
};

To the beginning of the terrain Sandcastle it works fine. Looking into a fix now.

[OmarShehata]

Ok, I got it. It was peculiar because, what you see there isn't imagery flipped. It's the wrong terrain geometry.

Now why would an upside down image cause it to load the wrong terrain tiles? It's because VRTheWorldTerrainProvider constructs geometry from a heightmap by reading the pixels! The fix is just to make sure flipY: false in the requestTileGeometry function. Opening a PR now.