[Android] Issue with background opacity with vulkan renderer when `display/window/per_pixel_transparency/allowed` is enabled

[m4gr3d]

Tested versions

• Reproducible in Address remaining feedback on Android background transparency #107473

System information

Reproducible on Samsung Tab S8 running Android 15; reproducible on Pixel 9 Pro Fold running Android 16

Issue description

#107473 is adding transparency support on Android.
While testing the capability using the Window Management demo project, I noticed that the viewport background rendering differs between the OpenGL and Vulkan renderers.

It behaves as expected using the OpenGL renderer as shown in the recording below:

screen-20250707-125750.mp4

Using the Vulkan renderer, when get_viewport().transparent_bg == false, the viewport background is not fully opaque as with the OpenGL renderer. See the recording below for the issue:

screen-20250707-130230.mp4

Steps to reproduce

• Grab Android templates from Address remaining feedback on Android background transparency #107473: https://github.com/godotengine/godot/actions/runs/16125750520?pr=107473
• Using the latest version of the editor (beta2 or master or a build from Address remaining feedback on Android background transparency #107473 should do fine), open the https://github.com/godotengine/godot-demo-projects/tree/master/misc/window_management project
• Set the project renderer to mobile, and export the project onto an Android device
• Switch the transparent toggle on/off to replicate the issue.

Minimal reproduction project (MRP)

https://github.com/godotengine/godot-demo-projects/tree/master/misc/window_management