drawBitmap
public void drawBitmap (Bitmap bitmap, Rect src, Rect dst, Paint paint)
src 从图片上取一个范围
new Rect(int left, int top, int right, int bottom);
从left,top点截取到right,bottom点大小的区域
dst 画到屏幕的范围
new Rect(int left, int top, int right, int bottom);
将图片填充left,top到right,bottom的区域
