alpha blend mode / add shader compatbility

since we are trying to avoid alpha sorting issues, we are looking to use additive alpha blending, which should work oder-independent.

now, in eevee you can achieve this by setting the material to alpha blend, and using an add shader (see attached image). unfortunately, this does not export to verge3d in the same way. the material will not blend additively.

as a workaround, we are trying to output the color directly, and setting the blend mode via script manually:
app.scene.getChildByName("BezierCircle.004").material.blending=v3d.AdditiveBlending;
while this works, it is not completely the same blend effect, and it also will not look correct in the blender viewport.

future support of the add shader with transparency would be much appreciated. alpha or multiply blend modes are very useful, as they do not depend on the draw order.