Change Opacity

[DanielJDufour]

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Users will want to change the opacity of the GeoTIFF layer displayed on the map

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• GeoRaster Layer for Leaflet currently does this: https://github.com/GeoTIFF/georaster-layer-for-leaflet-example/blob/master/examples/load-via-script-tag.html#L48
• The Layer base class for OL supports this. If ol-geotiff extends this base class then it should work by default: https://openlayers.org/en/latest/apidoc/module-ol_layer_Layer-Layer.html