Subsurface Scattering breaks per-pixel transparency blend modes

[GK-GreyGhost]

Godot version

4.0.2

System information

Windows 10, Vulkan / Forward+, Nvidia 2070 Super

Issue description

Enabling Subsurface Scattering on a material causes whole scene to use additive blending instead of mix for opaque materials and additive for transparent materials

Steps to reproduce

1. Enable display/window/per_pixel_transparency/allowed in the project settings
2. Set get_viewport().transparent_bg = true
3. DisplayServer.window_set_flag(DisplayServer.WINDOW_FLAG_TRANSPARENT, true, 0).
4. set WorldEnvironment background to Color(0,0,0,0)
5. create a mesh instance
6. add material to instance and enable SubsurfaceScattering

Minimal reproduction project

transparent_test.zip

[Calinou]

• Related to https://github.com/godotengine/godot/issues/67101 (same cause).

This likely occurs because the subsurface scattering shader writes to the final scene's alpha channel when it shouldn't.