Negative lights not working as expected

[Hevedy]

Godot version:
v3.2.2.stable (Mono version)

OS/device including version:
Windows 10 Pro
Intel i7 (Old Gen)
Nvidia 970GTX with 441.20 driver.

Issue description:
At using negative lights at any Tonemap other than linear one the negative lights don't work anymore (Only work on 1 of 4).
Affects all lights.

Steps to reproduce:
1.) Download the provided project.
2.) Check the first scene in the project and the projected light from the spotlight.

Minimal reproduction project:
ShowcaseIssues.zip

[clayjohn]

It looks like values are getting written below 0. I guess there needs to be a check to ensure that values are clamped at 0

[Yetizone]

Hello, I'm interested in working on this. I've downloaded the project and I see the light working as follows:

From what I'm understanding this behaviour is undesired.
What's the expected one ? Should the red circle in the middle not be there and instead just be black ?

[Hevedy]

Yeah should all black, the red circle happen because is overflow of the values outside of linear postprocess

[clayjohn]

@Yetizone set the tonemap mode to linear and you will see the desired outcome. It is just an all black circle.

[Yetizone]

I see, thanks for the tip !

[akien-mga]

Fixed by #42056.

[Yetizone]

This issue still happens in master. I think it should remain open for now.

I noticed the PR targeting 3.2 with a fix got merged so I'll write one for master now.