WINCE下的画圆弧算法，输入参数为hdc,x0,y0,r,a,b。x0,y0为圆心坐标，r为半径，a为起始角度（逆时针），b为b和a之间的夹角（逆时针）。采用离散的算法。-WINCE under an arc algorithm, the input parameters for hdc, x0, y0, r, a, b. x0, y0 as the center coordinates, r is the radius, a starting angle (counterclockwise)