Terrain skirts are causing artifacts in terrain shadows

[calogeromauceri]

When terrain shadows are enabled, the skirts contribute in casting shadows causing shadows artifacts.

This is better visible when the light source is tangent to surface (e.g. it is near the horizon).

I'm attaching a simple sandcastle example for demonstrating the current behavior: advance the time through the timeline for seeing the shadows artifacts caused by the skirts. Disabling the terrain skirts gets rid of some of those artifacts, but that of course has some other bad side effects, e.g. worse tiles edges connection

Sandcastle example:
Sandcastle

Ideally, I solid shadow should be casted when the light is low and the light source is occluded by a skirt "wall". Something like in the following animation

Browser:
any

Operating System:
any

[ggetz]

Thanks for pointing this out @calogeromauceri!

We're reviewing your suggested fix in #11460.