Display splatmap textures in World Machine

World Machine is often used to create splatmaps to guide the placement of tiled material textures on the terrain, instead-of / in-addition-to creating a single unique "world material".

However, it's hard to get a feel for what the result will look like since you cannot see the textures within WM itself.

Add the ability to view the resulting tiled materials distributed by a splatmap. For a bonus, allow a variant that uses an ID map instead (only a single material specified at any given point).