keep the orginanl image resoluting and support generating retina image

[haoqi0818]

now i can use this way to generate a final retina image, however i found that all of the image element become blurry.

html2canvas(el, {
    canvas: (function() {
        // 生成多倍图
        var ratio = 2;
        var canvas = document.createElement('canvas');
        canvas.width = document.documentElement.clientWidth * ratio;
        canvas.height = document.documentElement.clientHeight * ratio;
        var ctx = canvas.getContext('2d');
        ctx.scale(ratio, ratio);
        return canvas;
    })()
})

change function "CanvasRenderer.prototype.renderBackgroundRepeat" can fix this problem