add a removeBlackColor in colorLayerEffects

[gmaillet]

The removeWhiteColor is very convenient to merge a cartographic layer with an ortho layer, but a removeBlackColor could be useful to manage local or partial layers. For example a high resolution layer (or a newly updated layer) on a city or a region, with black on non-available areas.

Expected Behavior

if the intensity is less than a threshold, alpha becomes zero

Actual Behavior

Not available

Possible Cause/Fix/Solution

• add a new colorLayerEffects in LayeredMaterial.js
• add a applyBlackToInvisibleEffect in color_layers_pars_fragment.glsl