Viewport transparent background doesnt work

[nunodonato]

Operating system or device - Godot version: Linux, Godot Master

Issue description:
Rendering a viewport to a texture having the transparent flag on doesn't render a transparent image. There is always a grey background that occupies the entire viewport.
I tried with several different options (own world, setting up a new world environment, etc), but the background is always there.

[reduz]

I added this a few days ago, so closing

[nunodonato]

Still doesn't work for me.
Also related, it seems to be vertically flipped, regardless of that property value