ImageOverlay: Add support for custom handling of overlay texture

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When loading image overlays they are currently sorted and layered in the albedo channel. However there may be use cases that benefit from enabling manual processing or custom mapping of the loaded textures to materials:

• Mapping a texture value to another material channel (eg quant mesh water mask to specular channel)
• Custom shader transitions between overlays (eg over time, toggle, distance, etc)

One approach may look like so:

const overlay = new MVTImageOverlay( {
  url,
  layer: [ 'water' ],
  handleTexture: ( texture, material, object, tile ) => {
    material.specularMap = texture;

    // return "true" to prevent albedo layering
    return true;
  },
} );

cc @shotamatsuda

Start at the MVTImageOverlay entry point and trace how loaded image overlays are sorted and layered into the albedo channel. Define the custom texture-handling behavior described in the issue, including mapping to another material channel and preventing albedo layering, then verify the overlay cases and custom transitions.