Fix broken 2D light blending, addresses #49922

[bitbrain]

This pull request aims to address issues raised in #49922 - the problem was that the color was already modified with the modulate color. It was impossible to then rely on the original color for lighting. To fix the issue I made sure to remember the original color for the light processing.

Before this change

After this change

Bugsquad edit:

• Fixes Vulkan: Light2d Blend Modes do not behave correctly #49922

[h0lley]

Tested it in a project originally built in 3.x and can confirm it looks almost identical now 👍

I say almost because in my test scene here, the light source sprite (to which PointLight2D is a child) still looks different depending on CanvasModulate. this is presumably a different issue though.

[bitbrain]

@h0lley how does the above scene look in 4.0 without this fix?

[h0lley]

@h0lley how does the above scene look in 4.0 without this fix?

[clayjohn]

Looks good to me!

For future reference, the reason this PR is correct is that it multiplies the base color of the sprite with the lights color in calculating the lights contribution before blending. In Godot 3.x lights were each rendered in a separate pass where you would redraw all sprites, calculate their color normally, multiply by the light, then blend using a fixed function blending mode.

To achieve the equivalent in Godot 4.x we need to save the sprite's color and then multiply it by the light color for each light before blending.

[akien-mga]

Great work :)