Vulkan: Alpha channel not included in Viewport capture

Created by: supagu

Godot version: Vulkan

OS/device including version: Linux/Arch/Manajro

Issue description: I have a ViewportContainer with a Viewport child to render something in the viewport. I have a custom ShaderMaterial applied to the ViewportContainer with some code like this:

shader_type canvas_item;

uniform float width: hint_range(0.0, 10.0);
//uniform vec4 color: hint_color;

void fragment() {
	vec4 c = texture(TEXTURE, SCREEN_UV, 0.0);
        COLOR=c.aaaa;
}

the texture always comes out white.

what I am rendering in the viewport is a mesh with a material that is like so:

so that it draws in the alpha channel.

this worked fine when in the godot 3.2 branch and seems to have broken when I try it in Vulkan branch,