Punching alpha fillled holes to render-to-textures in Three.js
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I am using render-to-texture to do postprocessing and then blending several 2D layers together.

Currently I am using stencil mask to make "holes" in render-to-texture targets and leaving some of the areas transparent. However, this is little cumbersome in my case. I'd rather ignore the stencil mask and then just would use normal polyfill operations to draw the holes.

What kind of methods there exist for rendering "fill to alpha 0.0" areas in the scene? I.e. the existing rendet-to-texture destination alpha value would be ignored and just replaced with 0.0 value. I assume you can set OpenGL mode bits so (how?) that this can done, without the need of using a custom fragment shader.

I already know how to set depth mask to ignore mode, so I can redraw over the top of the existing polygons.
