Lit both sides equally for double-sided lambert material instances.

lambert-patch.js

THREE.ShaderLib[ 'lambert' ].fragmentShader = THREE.ShaderLib[ 'lambert' ].fragmentShader.replace(
    `vec3 outgoingLight = reflectedLight.directDiffuse + reflectedLight.indirectDiffuse + totalEmissiveRadiance;`,
    `
    #ifdef DOUBLE_SIDED
        reflectedLight.indirectDiffuse = getAmbientLightIrradiance( ambientLightColor );
        reflectedLight.indirectDiffuse *= BRDF_Diffuse_Lambert( diffuseColor.rgb );
        
        reflectedLight.directDiffuse = diffuseColor.rgb;
        reflectedLight.directDiffuse *= BRDF_Diffuse_Lambert( diffuseColor.rgb ) * getShadowMask();
    
        vec3 outgoingLight = (reflectedLight.directDiffuse + reflectedLight.indirectDiffuse + totalEmissiveRadiance).rgb * ( 1.0 - 0.45 * ( 1.0 - getShadowMask() ) );
    #else
        vec3 outgoingLight = reflectedLight.directDiffuse + reflectedLight.indirectDiffuse + totalEmissiveRadiance;
    #endif
    `
);