Add shadowed sky light

[mafiesto4]

Add support for using Global SDF to render shadow for Sky Light. This should reduce sky-light leaking into interiors and improve ambient lighting.

Global SDF usage in shaders: https://docs.flaxengine.com/manual/graphics/models/sdf.html#shaders

Example of raytracing in Screen Space Reflections shader:

FlaxEngine/Source/Shaders/SSR.shader

Lines 136 to 150 in 228ef4e

	// Fallback to Global SDF and Global Surface Atlas tracing
	#if USE_GLOBAL_SURFACE_ATLAS && CAN_USE_GLOBAL_SURFACE_ATLAS
	GlobalSDFTrace sdfTrace;
	float maxDistance = 100000;
	float selfOcclusionBias = GlobalSDF.CascadeVoxelSize[0];
	sdfTrace.Init(gBuffer.WorldPos + gBuffer.Normal * selfOcclusionBias, reflectWS, 0.0f, maxDistance);
	GlobalSDFHit sdfHit = RayTraceGlobalSDF(GlobalSDF, GlobalSDFTex, GlobalSDFMip, sdfTrace);
	if (sdfHit.IsHit())
	{
	float3 hitPosition = sdfHit.GetHitPosition(sdfTrace);
	float surfaceThreshold = GetGlobalSurfaceAtlasThreshold(GlobalSDF, sdfHit);
	float4 surfaceAtlas = SampleGlobalSurfaceAtlas(GlobalSurfaceAtlas, GlobalSurfaceAtlasChunks, RWGlobalSurfaceAtlasCulledObjects, GlobalSurfaceAtlasObjects, GlobalSurfaceAtlasDepth, GlobalSurfaceAtlasTex, hitPosition, -reflectWS, surfaceThreshold);
	result = lerp(surfaceAtlas, float4(result.rgb, 1), result.a);
	}
	#endif

[HydrogenC]

How is this meant to be implemented? I've tried to trace in the normal direction but it doesn't look good. It would be theoretically better to sample on the hemisphere but won't that be too computationally costly?

[mafiesto4]

I think it would make sense to sample part of the hemisphere around the surface normal to account for ambient light diffusion. We use surface normal for skylight lighting (eg. to sample sky texture). This skylight shadowing is meant to be very ambient (eg. when lighting scene with single skylight with HDRI image for overcast day lighting scenario) so we could do it in low-res with upscaling and some temporal effect (noise filtered by temporal accumulation).