This produces more accurate results for the `EmissiveStrengthTest` glTF test case.
(Requires manually setting the emission, for now)
Before: <img width="1392" alt="Screenshot 2023-03-04 at 18 21 25" src="https://user-images.githubusercontent.com/418473/222929455-c7363d52-7133-4d4e-9d6a-562098f6bbe8.png">
After: <img width="1392" alt="Screenshot 2023-03-04 at 18 20 57" src="https://user-images.githubusercontent.com/418473/222929454-3ea20ecb-0773-4aad-978c-3832353b6871.png">
Tagging @JMS55 as a co-author, since this fix is based on their experiments with emission.
# Objective
- Have more accurate results for the `EmissiveStrengthTest` glTF test case.
## Solution
- Make sure we send the emissive color as linear instead of sRGB.
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## Changelog
- Emission strength is now correctly interpreted by the `StandardMaterial` as linear instead of sRGB.
## Migration Guide
- If you have previously manually specified emissive values with `Color::rgb()` and would like to retain the old visual results, you must now use `Color::rgb_linear()` instead;
- If you have previously manually specified emissive values with `Color::rgb_linear()` and would like to retain the old visual results, you'll need to apply a one-time gamma calculation to your channels manually to get the _actual_ linear RGB value:
- For channel values greater than `0.0031308`, use `(1.055 * value.powf(1.0 / 2.4)) - 0.055`;
- For channel values lower than or equal to `0.0031308`, use `value * 12.92`;
- Otherwise, the results should now be more consistent with other tools/engines.
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