WebGLRenderer: Setting alpha to false by default.

WebGL itself has it set to true by default, but I have the feeling alpha is not needed in most of the cases. Plus we save memory and increase performance.

src/renderers/WebGLRenderer.js

@@ -15,7 +15,7 @@ THREE.WebGLRenderer = function ( parameters ) {
 
 	_precision = parameters.precision !== undefined ? parameters.precision : 'highp',
 
-	_alpha = parameters.alpha !== undefined ? parameters.alpha : true,
+	_alpha = parameters.alpha !== undefined ? parameters.alpha : false,
 	_premultipliedAlpha = parameters.premultipliedAlpha !== undefined ? parameters.premultipliedAlpha : true,
 	_antialias = parameters.antialias !== undefined ? parameters.antialias : false,
 	_stencil = parameters.stencil !== undefined ? parameters.stencil : true,