viewport texture pre-multiplied alpha values differ between compatibility and forward+ (compatibility is correct)

### Tested versions

reproducible in 4.3 stable

### System information

Godot v4.3.stable - Windows 10.0.19045 - Vulkan (Forward+) - dedicated NVIDIA GeForce GTX 1650 (NVIDIA; 32.0.15.5612) - Intel(R) Core(TM) i3-10100 CPU @ 3.60GHz (8 Threads)

### Issue description

when using a viewport texture with transparency, on a material with pre-multiplied alpha enabled, colours are not correctly pre-multiplied in forward+ mode, but are correct in compatibility.


if this is considered a duplicate of #88603, please keep this report as it is more accurate/concise

### Steps to reproduce

example shows a viewport texture rendering a flat grey colour with gradient transparency, then overlaid back over the same grey.

view the example in 2d mode. in forward+ there is a visible gradient where there should be a flat grey. switch to compatibility and it is flat

### Minimal reproduction project (MRP)

[premult_issue_v2.zip](https://github.com/user-attachments/files/16738075/premult_issue_v2.zip)

Import and run the linked premult_issue_v2.zip minimal reproduction project, then compare its viewport-texture output in Forward+ and Compatibility rendering modes. Trace how viewport textures and pre-multiplied alpha are handled in the two paths; done means the Forward+ result matches Compatibility, with a flat grey overlay and no unintended gradient.

Written by the indexing model from the issue text.