Configurable alpha parameter for webgl construction

[pandamicro]

Re: cocos-creator/fireball#5965

Changes proposed in this pull request:

• Configurable alpha parameter for webgl construction

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@cocos-creator/engine-admins

[jareguo]

建议加上：“但是这样有可能导致叠加一些混合模式时渲染异常”

[pandamicro]

你也没办法说清楚是什么情况下异常，我觉得还是不要加了，反正 alpha 为 false 的情况表现和 true 就是不同的，具体怎么不同都是跟 webgl 有关系，跟我们引擎没关系

[jareguo]

嗯