拓展的Bresenham画直线（全象限全斜率），采用实时判断斜率的方式分别对正负斜率进行处理，用了对称的思路做出图象。效果很好，代码非常简洁明了。,Bresenham draw a straight line extended (all-quadrant all-slope), the use of real-time manner to determine the slope of the positive and negative slope, respectively, to deal with the idea of symmetry to the image. Very good, very concise code.