blend_mul can no longer make an image brighter

[rainlizard]

Godot version

v4.0.beta2.official [f8745f2]

System information

Windows 10, default, GTX 970

Issue description

blend_mul can no longer make an image brighter, and negative alpha values no longer work (I think it functioned like blend_mul).

Steps to reproduce

shader_type canvas_item;
render_mode blend_mul;

void fragment() {
	vec4 baseCol = texture(TEXTURE, UV);
	baseCol.r *= 5.0;
	baseCol.g *= 5.0;
	baseCol.b *= 5.0;
	baseCol.a *= 1.0;
	COLOR = baseCol;
}

Minimal reproduction project

blend_mul 3.5.zip
blend_mul 4.0.zip

[Calinou]

This likely happens because in 4.0, vertex color in 2D and 3D is limited to 8 bpc and can no longer be "overbright". You can notice this if you set Modulate to Color(2, 2, 2) for instance (using Raw mode on the color picker).

I think this should be made possible again, but it may have to be an opt-in project setting for performance reasons.

[rainlizard]

I'm having an absolute impossible time recreating some layered effects from 3.5 without this. blend_add is no substitute, it looks ghastly. I managed to recreate blend_mul using SCREEN_TEXTURE and SCREEN_UV but ran into limitations of SCREEN_TEXTURE not working when used multiple times.

I hope there's desire from others for this feature, I tried compiling Godot myself but it took over 30 minutes to compile and have no idea where to even look to change something like this.