This function is used to set current aspect ratio.
void setaspectratio( float xasp, float yasp );
The x aspect factor. For example, draw a rectangle with a width of 100, and the actual drawing width is 100 * xasp.
The y aspect factor. For example, draw a rectangle with a height of 100, and the actual drawing height is 100 * yasp.
If the aspect radio is negative, you can flip the axes. For example, after performing setaspectratio(1, -1), you can make the y axis positive.