Forward+ Window transparency is not working, works in Compatibility renderer

### Tested versions

Reproducible in 4.3 all dev snapshots, 4.2 and 4.1

### System information

Godot v4.3.dev4 - Windows 10.0.19045 - Vulkan (Forward+) - dedicated AMD Radeon RX 6900 XT (Advanced Micro Devices, Inc.; 31.0.24002.92) - AMD Ryzen 9 7950X 16-Core Processor (32 Threads)

### Issue description

Transparency does not work in the Forward+ renderer, at least on my system. It works fine in Compatibility renderer.

I set the correct parameters for transparency settings and I have used the same script on all the versions I tested:
4.3 all dev snapshots, 4.2 and 4.1

Project settings advanced enabled
project>project settings>Display > Window > per pixel transparency> enabled
project>project settings>Display > Window > transparent > enabled

Not sure if this an AMD issue.

### Steps to reproduce

Enable Transparency settings in project settings:

Project settings advanced enabled
project>project settings>Display > Window > per pixel transparency> enabled
project>project settings>Display > Window > transparent > enabled

Create any type of node and add use a simple script to set the window and viewport to be transparent:

``` gdscript
func _ready() -> void:
get_tree().get_root().set_transparent_background(true)
DisplayServer.window_set_flag(DisplayServer.WINDOW_FLAG_TRANSPARENT, true, 0)

```

The window should be transparent, if not, try switching to Compatibility renderer.

### Minimal reproduction project (MRP)

[Bug Sample project files](https://github.com/godotengine/godot/files/14505046/BugTest.zip)

Start with the linked Bug Sample project and reproduce the behavior using the listed Display > Window transparency settings and the provided GDScript entry point. Compare Forward+ with the Compatibility renderer on the reported Windows and AMD setup, then verify that the sample window becomes transparent in Forward+ as well.

Written by the indexing model from the issue text.