GLTF: Fix wrong color space for GLTFLight on export

[aaronfranke]

Oh my goodness, I wrote this 2 months ago and forgot to submit it as a PR... 🤦‍♂️

glTF uses linear color space for lights, while Godot's Light3D node uses sRGB color space (the docs don't specify, but I tested how it visually looks, and well, it's either sRGB or close to it).

In the past I had only tested lights with trivial colors, such as pure white or pure red. Or maybe I've only tested importing lights, and not exporting them. Godot performs a color space conversion during import, but does not perform the reverse on export. This means that the color of lights does not survive a round-trip. Additionally, the conversion was performed at the wrong step, leading to the GLTFLight data not matching the data in the glTF light. This PR fixes both of these problems. Now the color space is correctly converted when generating new nodes and when converting existing nodes.

To test this, I made a Light3D node and set its color to (0.75, 0.5, 0.25). In the current master, it incorrectly exports the Light3D color value as-is without performing a conversion, resulting in this file that doesn't visually match Godot:

{
    "asset": { "version": "2.0" },
    "extensions": {
        "KHR_lights_punctual": {
            "lights": [{ "color": [0.75, 0.5, 0.25], "type": "point" }]
        }
    },
    "extensionsUsed": ["KHR_lights_punctual"],
    "nodes": [
        {
            "extensions": { "KHR_lights_punctual": { "light": 0 } },
            "name": "OrangeLight"
        }
    ],
    "scene": 0,
    "scenes": [{ "nodes": [0] }]
}

With this PR, exporting the same scene gives almost the same file but with a color of [0.522521495819092, 0.214041143655777, 0.0508760772645473], which is the correct linear version of the sRGB (0.75, 0.5, 0.25), and then after import Godot reads the color as (0.75, 0.5, 0.25) again. With this PR, the exported glTF looks the same in Godot as it does in online glTF viewers such as glTF Viewer and glTF Sample Viewer.

Additionally, this PR adds the above color space information to the Light3D and GLTFLight documentation.

[fire]

I don’t think srgb with overbright is called srgb, but I think the idea is good. Need to fully review this.