Use additive blending for suns in ProceduralSkyMaterial to fix dark suns

[Calinou]

This prevents dark DirectionalLight3D colors from creating splotches in the sky, which is a commonly encountered issue when working on day/night cycle systems.

This affects how bright suns are rendered too, with a greater impact on heavily saturated colors.

If we consider the visual difference to be too important, we'll probably want to add a Sun Blend Mode property to control this (although ProceduralSkyMaterial is already quite complex as it is). If we want a compatible solution that still addresses the original issue, we could perform premultiplied alpha blending instead, with the "alpha" value being determined by the color's HSV value.

• See Add additional blending modes for the sun in ProceduralSkyMaterial godot-proposals#13748.

Testing project: test_sky_sun.zip

Preview

The scene has 4 suns.

Before	After

[DarkKilauea]

I tested this with #94496 to see if it fixed the sun looking too dim on HDR displays. It does appear to solve that issue, so I think this PR may be important to merge in 4.7 with the rest of the HDR related PRs.