Returns a list
[red, green, blue] of the RGB values of the pixel at
position (x,y). Each value is a number in the range 0-255
indicating the intensity of the corresponding RGB color. These
numbers can be turned into a color string using the
color_rgb function (see next section).
Note that pixel position is relative to the image itself, not the
window where the image may be drawn. The upper-left corner of the
image is always pixel (0,0).