Strange effect with shadows enabled on DirectionalLight

[eon-s]

Godot version: master 32afcbc but more recent too

Issue description:
Using default settings, when turning shadows on on a DirectionalLight, the editor get some weird effect on textures.
Camera viewport get less noisy output and the running camera even less, but is still there.

I think the effect is not present on OmniLight and just at some extent on SpotLight (faces parallel to the light direction).
This may be a bad default setting issue, but not sure.

Steps to reproduce:
Add a directional light to a 3D scene and enable shadows.

Link to minimal example project:

shadowtest.zip

[Zireael07]

Looks very close to my weird flashes from #8651, so chances are the two are related.

[eon-s]

This problem doe not exists in a late january build (just the problem that every gizmo produces shadow).

[reduz]

hmm this looks like default shadow bias setting, should investigate

[eon-s]

A normal bias (not shadow) of around 0.7 make the strips disappear but not sure if is the correct setting.

[reduz]

Rewrote the way bias works in DirectionalLights, this should make it work much better. Also changed default settings so it looks good on the first attempt