Configurable StandardMaterial shadowcaster behavior

[atlv24]

What problem does this solve or what need does it fill?

Transparent StandardMaterial meshes are being rendered in shadow caster passes whenever base_color.a > 0.05

What solution would you like?

Either make the 0.05 configurable, or make it a per-material option on whether shadows are cast or not. Perhaps a flag for both cast/receive.

What alternative(s) have you considered?

Using NotShadowCaster. Its annoying because it doesnt stay synchronized to the material.

Transparent shadow maps: https://wickedengine.net/2018/01/18/easy-transparent-shadow-maps/

Additional context

0.05 cutoff was added here #7784 (comment)

[superdump]

Having a global default that is overridable by materials seems reasonable to me, a bit like the alpha mask threshold (though that doesn't really have a global default.)