Values lower than 1.0 can be used to make the fog rendering not fully
obstruct the sky. This can be desired when using fog as a purely
atmospheric effect, without intending to use fog for open world fog
fading.
When set to 0.0, fog rendering behavior will be similar to Godot 3.x
where sky rendering was never affected by fog.
This allows light sources to be specified in physical light units in addition to the regular energy multiplier. In order to avoid loss of precision at high values, brightness values are premultiplied by an exposure normalization value.
In support of Physical Light Units this PR also renames CameraEffects to CameraAttributes.
Per-light energy gives more control to the user on the final result of
volumetric fog. Specific lights can be fully excluded from volumetric fog
by setting their volumetric fog energy to 0, which improves performance
slightly. This can also be used to prevent short-lived dynamic effects
from poorly interacting with volumetric fog, as it's updated over several
frames by default unless temporal reprojection is disabled.
Volumetric fog shadows now obey Light3D's Shadow Opacity property as well.
The shadow fog fade property was removed as it had little visible impact
on the final scene's rendering.
This can be used to make shadows translucent for a specific light.
The light distance fade system also uses this to smoothly fade the shadow
when the light fade transition distance is greater than 0.
