Add emissive_exposure_weight to the StandardMaterial

[geckoxx]

Objective

• The emissive color gets multiplied by the camera exposure value. But this cancels out almost any emissive effect.
• Fixes Emissive materials not displayed properly when imported from gltf files #13133
• Closes PR Ignore emissive_light when multiplying with exposure #13337

Solution

• Add emissive_exposure_weight to the StandardMaterial
• In the shader this value is stored in the alpha channel of the emissive color.
• This value defines how much the exposure influences the emissive color.
• It's equal to Google's Filament:
  https://google.github.io/filament/Materials.html#emissive
  https://github.com/google/filament/blob/4f021583f1c721486baaa9291be5943216c244ec/shaders/src/shading_lit.fs#L287

Testing

• The result of EmissiveStrengthTest with the default value of 0.0:

without bloom:

with bloom:

[BD103]

Looks good and tested!

cc @mockersf, you may be interested in this as it may affect screenshot tests.

[pcwalton]

Good catch. We should definitely add this to the migration guide as it will surprise people.

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.