Experimental effect AlphaDiscard for 3DObject Billboard objects

Experimental effect for 3DObject billboard objects (e.g. a single plane with a texture) to help with transparency conflicts around transparent billboard edges overlapping with other objects.

It must be the _only_ effect on the 3DObject and you cannot use the normal color and opacity properties of the object, instead use the color and opacity parameters of the effect if needed.

Use threshold to control alpha gradient cutoffs. Experiment! Let me know if it works for you.

In my experience, it will not help with 3D Shape billboards, likely due to 3D Shape being rendered to an intermediate frame buffer before rendering.

To create gltf billboard models, check out this blender addon: https://docs.blender.org/manual/en/latest/addons/import_export/images_as_planes....

Also see the plane.gltf, which is a simple textured plane model to use a billboard model.

If you have z-fighting issues, you might need to do something a little different, you'll need to displace objects that are right on top of each other by a small amount (check if another object's x,y,z is very close and displace by a small amount.)