Cast Shadows Flag: MIS sampling needs to determine the shadow ray from the env sample direction rather than new sample

[gkjohnson]

Related to #274

See here:

three-gpu-pathtracer/src/materials/PhysicalPathTracingMaterial.js

Lines 891 to 902 in 1bb778b

	vec3 attenuatedColor;
	if (
	envPdf > 0.0 &&
	isDirectionValid( envDirection, normal, faceNormal ) &&
	! attenuateHit( bvh, rayOrigin, envDirection, bounces - i, isShadowRay, attenuatedColor )
	) {
	// get the material pdf
	vec3 sampleColor;
	float envMaterialPdf = bsdfResult( outgoing, clearcoatOutgoing, normalize( invBasis * envDirection ), normalize( clearcoatInvBasis * envDirection ), surfaceRec, sampleColor );
	bool isValidSampleColor = all( greaterThanEqual( sampleColor, vec3( 0.0 ) ) );
	if ( envMaterialPdf > 0.0 && isValidSampleColor ) {

Maybe it makes sense to be able to determine the specular PDF without getting the full set of material properties.