Add material for procedural windows on OSM buildings

[argallegos]

This PR adds two materials for procedural windows on OSM buildings. For an overview of these materials and discussion, see #36.

In this PR:

• MI_ProceduralWindowsMetadata, a material instance that adds windows and randomizes building color based on metadata.
• MI_ProceduralWindowsSimple, a material instance that adds windows.
• An example scene for each version of the material
• MF_TriplanarWindows, the material function that applies the window texture.
• Two Material Layers, one for each material instance
• A Material Layer Blend
• A tiling window texture

If we decide to add this to the Samples, prior to merging we should determine whether to create a full sample scene in CesiumSamples/Maps, or leave the sample scenes as they are in CesiumSamples/Materials/ProceduralWindows. I think it's important to include at least the Metadata scene, because in order for the material to work correctly, the Tileset actor must be set up properly with the CesiumEncodedMetadata component.

[kring]

This looks awesome @argallegos! I'm all for turning this into a proper sample map.

[argallegos]

This is now ready for review. I created a proper sample scene (10_CesiumOSMWindows_Metadata) with a Getting Started note. I deleted the example scene for the simple version of the material - I didn't feel it was necessary.

Also, I noticed an issue in the 09 sample scene - the "Exclude Tiles Inside" settings weren't configured properly on Cesium World Terrain, so the custom watermask tiles were getting culled out. This PR includes that minor fix.

[kring]

This is really cool @argallegos, nice job!

Only small thing I noticed is that when using MI_ProceduralWindowsMetadata, adding a raster overlay doesn't do anything. This isn't a problem, really, but I thought I'd mention it because there's a layer for raster overlays in the material instance. Might as well remove that and make the material simpler. MI_ProceduralWindowsSimple does work with a raster overlay, though. In any case, any changes here can be a separate PR.

We should definitely mention and screenshot this new sample in the release roundup this month @slchow!