The parent `_validate_property()` wasn't called, which led to shadow
properties being visible even if shadows were disabled on a
DirectionalLight3D node.
Some split distance properties are unused depending on the
current shadow mode. Also, Blend Splits can only be used if the shadow
mode is PSSM 2 Splits or PSSM 4 Splits.
This also moves the Fade Start property to be located after the
split properties. This avoids intertwining "conditional" properties
with a property that's always available.
OmniLight3D:
* Fixed lack of precision in cube map mode by scaling the projection's
znear.
* Fixed aliasing issues by making the paraboloids use two square regions instead of two half
squares.
* Fixed shadowmap atlas bleeding by adding padding.
* Fixed sihadow blur's inconsistent radius and unclamped sampling.
SpotLight3D:
* Fixed lack of precision by scaling the projection's znear.
* Fixed normal biasing.
Both:
* Tweaked biasing to make sure it works out of the box in most situations.
* Simplified code a lot, bias based on normalized cascade size.
* Lets scale cascades, max distance, etc. without creating acne.
* Fixed normal biasing in directional shadows.
I removed normal biasing in both omni and spot shadows, since the technique can't be easily implemented there.
Will need to be replaced by something else.
The Optimized shadow depth range was removed in late 2020 in favor
of the Stable shadow depth range, but it still had a (broken) property
that allowed to enable it.
