Add "alphaMask" option for ImageOverlays

[gkjohnson]

Related to #1368
Fix #1365

• Adds "alphaMask" option for "ImageOverlays". When "true" the color of the layer is ignored and alpha is applied globally to all layers below this layer.
• Adds "alphaTest" option for "ImageOverlays", which works similarly to three.js' "Material.alphaTest" option. When "0" no adjustment to alpha is done. When "true" alpha is clipped to [ 0, 1 ] with antialiasing for a clean mask an compatibility with alpha to coverage.
• Adds "alphaInvert" option which inverts the alpha of the given layer.

Note

• In order for the alpha to apply correctly the base materials have to be pre-configured to support alpha, in other words "transparent" or "alphaToCoverage" must be set to "true" by the user.
• In order to render both "color" geojson (or other) overlays in addition to an alpha mask you will have to use two different layers.

TODO

• Convert the new options to #defines and javascript-expanded loop to avoid additional if-statements.
• Consider approach for using the "red" channel as alpha so mask overlays can use a smaller texture size.

cc @SoftwareMechanic - I decided it was easier to make a separate PR, but I think this covers everything and should be a simpler change. Let me know what you think and if it looks good I'll handle the TODO items and get this merged.