Replaced implicit emissive weight with default.

[dragostis]

Since StandardMaterial::emissive_exposure_weight does not get packed into the gbuffer in the deferred case, unpacking uses an implicit default value for emissive's alpha channel.

This resulted in divergent behavior between the forward and deferred renderers when using standard materials with default emissive_exposure_weight, this value defaulting to 0.0 in the forward case and 1.0 in the other.

This patch changes the implicit value in the deferred case to 0.0 in order to match the behavior of the forward renderer. However, this still does not solve the case where emissive_exposure_weight is not 0.0.

[pcwalton]

Seems fine, not ideal but an improvement on what we have.

[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.

[janhohenheim]

Agreed, it's at least better than the status quo

[JMS55]

I don't know that we should merge this until #14599 is finished.

[Soulghost]

I don't know that we should merge this until #14599 is finished.

It's fine. If this code is merged before #14599 , I can premultiply the lightmap_data with view_bindings::view.exposure to resolve the confliction. On the other hand, this PR will require some adaptation work.