anti_aliasing Figure's property smoothing the stair-like aspect when rendering curves

## Description

When some curves or parts of curves have a small slope with respect to the horizontal or vertical direction of ranges of screen pixels, their raw rendering shows a stair-like aspect.

Figures have an `.anti_aliasing` property that can be used to smooth this rendering effect. Possible values are `'off'`, `'2x'`, `'4x'`, `'8x'`, or `'16x'`.

Enabling the anti-aliasing decreases the rendering speed of graphics. In average, the time spent to build and display graphics varies approximately according to the following factors:

.anti_aliasing | off | 2x | 4x | 8x | 16x
time spent | 1 | 1.4 | 2.1 | 3.6 | 6.6

## Examples

    x = [0 1];
    y = (0:0.025:1)';
    y = [y y+0.02]';

    th = 0:2:360;
    xe = 4*cosd(th); xe = (0.1:0.05:1)' * xe;
    ye = sind(th);   ye = (0.1:0.05:1)' * ye;

    usecanvas(%t);

    i = 0;
    for aa = ['off' '4x' '16x']
        f = scf(i);
        f.axes_size = [300 600];
        f.figure_position(1) = 100+300*i;
        f.anti_aliasing = aa;  // <<<<<<<<
        clf
        subplot(2,1,1)
        plot2d(x,y)
        subplot(2,1,2)
        plot2d(xe',ye')
        title(".anti_aliasing = ''"+aa+"''", "fontsize",4);
        i = i+1;
    end

## See also

- Figure properties
- usecanvas
- lighting
- pixel_drawing_mode
- system_setproperty