AmbientLight::affects_lightmapped_meshes doesn't work with deferred rendering

[alice-i-cecile]

NOTE: This only makes the setting work on the forward renderer. Making it work on the deferred renderer would probably require encoding more information in the g-buffer or similar. Please advise if I missed some obvious way to get it working on deferred.

By @marlyx in #20083

Spun out to make sure that this bug / context isn't lost to the sands of time :)