Vulkan: Background sky is still partially visible with transparent background enabled

[Calinou]

Godot version

4.0.beta2

System information

Fedora 36, Vulkan Forward Plus, AMD Radeon RX 6900 XT, KDE + KWin

Issue description

Note: This might be a KWin bug (or design decision), but I'm filing it here just in case. One possible cause is that KDE uses premultiplied alpha, while Godot doesn't. The background sky is not black, so even if it's fully transparent, its color is added instead of mixed.

---

This is what I see on the window (compositing is enabled). I can partially see the background sky:

Same window, but overlaid on a pure black background:

If I use KDE's screenshot tool to take a screenshot of the window, I get this:

The image outputted by MovieWriter (i.e. what Godot "sees") looks like this:

Steps to reproduce

• Run the attached project with movie maker mode enabled (click the "movie reel" icon in the top-right corner before running the project).
• Compare what you see on the window and the PNG images generated in the output/ folder.

Minimal reproduction project

test_movie_maker_png_transparent.zip

[Calinou]

I've noticed that there is a bit of code to clear alpha in OpenGL every frame when per-pixel transparency is enabled, but not in Vulkan:

https://github.com/godotengine/godot/blob/2a04b18d37de6c6e621db5a9dfd1cd0da5ccb015/platform/linuxbsd/x11/gl_manager_x11.cpp#L311-L319

cc @bruvzg