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;
         }