CanvasRenderer/SoftwareRenderer: Added support for new alpha property.

Context: https://code.google.com/p/chromium/issues/detail?id=234297
This gives a couple of extra fps :)

examples/js/renderers/SoftwareRenderer.js

@@ -4,12 +4,16 @@
  * @author mraleph / http://mrale.ph/
  */
 
-THREE.SoftwareRenderer = function () {
+THREE.SoftwareRenderer = function ( parameters ) {
 
 	console.log( 'THREE.SoftwareRenderer', THREE.REVISION );
 
+	parameters = parameters || {};
+
 	var canvas = document.createElement( 'canvas' );
-	var context = canvas.getContext( '2d' );
+	var context = canvas.getContext( '2d', {
+		alpha: parameters.alpha === true
+	} );
 
 	var shaders = {};
 

src/renderers/CanvasRenderer.js

@@ -23,7 +23,9 @@ THREE.CanvasRenderer = function ( parameters ) {
 	_canvasWidthHalf = Math.floor( _canvasWidth / 2 ),
 	_canvasHeightHalf = Math.floor( _canvasHeight / 2 ),
 	
-	_context = _canvas.getContext( '2d' ),
+	_context = _canvas.getContext( '2d', {
+		alpha: parameters.alpha === true
+	} ),
 
 	_clearColor = new THREE.Color( 0x000000 ),
 	_clearAlpha = 0,