Added camera alpha for webgl

Author: bitnenfer

v3/src/checksum.js

@@ -1,4 +1,4 @@
 var CHECKSUM = {
-build: 'e049e450-5c1f-11e7-9524-ad80f9e5c5d2'
+build: '60b859e0-5c21-11e7-a554-cbd3f53fc69e'
 };
 module.exports = CHECKSUM;

v3/src/renderer/webgl/WebGLRenderer.js

@@ -323,10 +323,14 @@ WebGLRenderer.prototype = {
         if (camera.backgroundColor.alphaGL > 0)
         {
             var color = camera.backgroundColor;
-
-            gl.clearColor(color.redGL, color.greenGL, color.blueGL, color.alphaGL);
-
-            gl.clear(gl.COLOR_BUFFER_BIT);
+            var quadBatch = this.quadBatch;
+            quadBatch.bind();
+            quadBatch.add(
+                camera.x, camera.y, camera.width, camera.height, 
+                color.redGL, color.greenGL, color.blueGL, color.alphaGL
+            );
+            quadBatch.flush();
+            this.currentRenderer.bind();
         }
 
         var list = children.list;