Vulkan: FXAA makes Transparent Bg to black

[volzhs]

Godot version:

3.2.4 beta 99fdf25

OS/device including version:

RX 570X / driver=amdgpu / GLES2 and GLES3

Issue description:

FXAA makes Transparent Bg to black. it happens on both gles2 and gles3.
MSAA works fine.

Steps to reproduce:

1. setting up a scene like below
   
2. enable Transparent Bg option
3. try Fxaa on and off
   

Minimal reproduction project:

fxaa_transparent_bg.zip

[clayjohn]

Unfortunately, the fix for this is going to be disabling FXAA when transparent background is enabled just like the other post processing options. :(

[Calinou]

Out of curiosity, has any other engine gotten transparent FXAA to work? Not that we particularly need it, but it's a nice thing to know.

[volzhs]

then, it might be a documentation issue.
@clayjohn it would be enough FXAA option be disabled in editor with a little documentation of fxaa option.

[Calinou]

This was confirmed on 4.0.alpha by @TuxTheAstronaut on the Godot Contributors Chat.

[clayjohn]

Despite my above comment, this should be fixed in 3.5 by #54585