[4.13] How to write alpha channel when rendering to RenderTarget from Blueprints

[RVillani (Rod Villani)]

##In 4.17 it worked by:

• Setting Draw material to AlphaComposite blend mode;
• Using standard Alpha (not inverted)
• Setting the mesh/widget material that display the RenderTexture to use inverted alpha output from RenderTexture (OneMinus node)
• Clear RenderTexture to (0, 0, 0, 1)