Use proper imports instead of using the "THREE" alias internally

Using the THREE alias makes things fail when trying to use three.js
with e.g. webpack which will assign some alias other than "THREE" to
the module.

src/Three.js

@@ -50,6 +50,7 @@ export { AudioLoader } from './loaders/AudioLoader.js';
 export { SpotLightShadow } from './lights/SpotLightShadow.js';
 export { SpotLight } from './lights/SpotLight.js';
 export { PointLight } from './lights/PointLight.js';
+export { RectAreaLightShadow } from './lights/RectAreaLightShadow.js';
 export { RectAreaLight } from './lights/RectAreaLight.js';
 export { HemisphereLight } from './lights/HemisphereLight.js';
 export { DirectionalLightShadow } from './lights/DirectionalLightShadow.js';

src/lights/RectAreaLightShadow.js

+import { LightShadow } from './LightShadow';
+import { PerspectiveCamera } from '../cameras/PerspectiveCamera';
+
 /**
  * @author aallison / http://github.com/abelnation
  */
 
-THREE.RectAreaLightShadow = function () {
+function RectAreaLightShadow() {
 
-	THREE.LightShadow.call( this, new THREE.PerspectiveCamera( 50, 1, 0.5, 500 ) );
+	LightShadow.call( this, new PerspectiveCamera( 50, 1, 0.5, 500 ) );
 
 };
 
-THREE.RectAreaLightShadow.prototype = Object.create( THREE.LightShadow.prototype );
-THREE.RectAreaLightShadow.prototype.constructor = THREE.RectAreaLightShadow;
+RectAreaLightShadow.prototype = Object.assign( Object.create( LightShadow.prototype ), {
+
+	constructor: RectAreaLightShadow,
 
-THREE.RectAreaLightShadow.prototype.update = function ( light ) {
+	update: function ( light ) {
 
 	// TODO (abelnation): implement
 
-};
+	}
+
+});
+
+export { RectAreaLightShadow };

src/renderers/WebGLRenderer.js

@@ -3,6 +3,7 @@ import { _Math } from '../math/Math';
 import { Matrix4 } from '../math/Matrix4';
 import { DataTexture } from '../textures/DataTexture';
 import { WebGLUniforms } from './webgl/WebGLUniforms';
+import { UniformsLib } from './shaders/UniformsLib';
 import { UniformsUtils } from './shaders/UniformsUtils';
 import { ShaderLib } from './shaders/ShaderLib';
 import { LensFlarePlugin } from './webgl/plugins/LensFlarePlugin';
@@ -1959,8 +1960,8 @@ function WebGLRenderer( parameters ) {
 			// RectAreaLight Texture
 			// TODO (mrdoob): Find a nicer implementation
 
-			if ( m_uniforms.ltcMat !== undefined ) m_uniforms.ltcMat.value = THREE.UniformsLib.LTC_MAT_TEXTURE;
-			if ( m_uniforms.ltcMag !== undefined ) m_uniforms.ltcMag.value = THREE.UniformsLib.LTC_MAG_TEXTURE;
+			if ( m_uniforms.ltcMat !== undefined ) m_uniforms.ltcMat.value = UniformsLib.LTC_MAT_TEXTURE;
+			if ( m_uniforms.ltcMag !== undefined ) m_uniforms.ltcMag.value = UniformsLib.LTC_MAG_TEXTURE;
 
 			WebGLUniforms.upload(
 				_gl, materialProperties.uniformsList, m_uniforms, _this );