Glow post effect is broken for softlight and screen blend modes (regression from 5567e89)

[fracteed]

Operating system or device - Godot version:
Windows 10 and Linux Mint

Godot 3.0

Issue description:
The glow post effect is giving an incorrect result for screen and softlight blend modes

Steps to reproduce:
turn on glow and use screen or softlight

Link to minimal example project:
glow_bug.zip

[himaghnam]

So that is the problem of glow! I thought it was light that was misbehaving!

[Gibbz]

same issue here. looks like the shader needs some clamping to preventing it going back into the negatives...

[ghost]

Issue first occurs in 5567e89