绘制QQ圆形图像

思路：这里用到的是图像求交。现在画布上画上圆形，即所需要的圆形头像，然后设置paint的属性设置为求交集，再将bitmap划到canvas上面就好了。

代码如下：

    private static Bitmap getCircleBitmap(Bitmap bitmap) {
            int width = bitmap.getWidth();
            int height = bitmap.getHeight();
            int r = width > height ? height : width;
            Bitmap bmp = Bitmap.createBitmap(width, height, bitmap.getConfig());
            Canvas canvas = new Canvas(bmp);
            Paint paint = new Paint();
            paint.setAntiAlias(true);
            RectF rect = new RectF(0, 0, r, r);
            canvas.drawRoundRect(rect, r / 2, r / 2, paint);
            paint.setXfermode(new PorterDuffXfermode(Mode.SRC_IN));
            canvas.drawBitmap(bitmap, null, rect, paint);
            return bmp;
        }