Subviewport premultiplied alpha still different

[AlexMitchellMus]

Godot version

#5993209b

System information

Arch Linux, GeForce RTX 3090 driver: nvidia v: 520.56.06

Issue description

Alpha handling inside a subviewport still looks different to the same shader outside of one, even when using blend_premul_alpha

Example project recreated from #17574 with Godot4

Steps to reproduce

Place objects with alpha inside a subviewport, set the subviewport to transparent background.

Set objects shaders to use blend_premul_alpha

Compare the object outside of the subviewport to that inside.

Minimal reproduction project

premult_subviewport.zip

[AlexMitchellMus]

Problem didn't exist, I had to set the size of the viewport correctly.