Type an exact zoom value, and view the drawing at its real size

Suggested by u/User_Number-1 on Reddit.

The zoom percentage can only be stepped up and down, and it stops at 800%.

We are adding a menu on the percentage itself: a field to type an exact value, steps up to 3200%, and a "Real size" entry. A browser never reports the physical size of a display, so real size is set once per device, by laying a bank card on a rectangle and adjusting it until it matches. A centimetre drawn then measures a centimetre under a ruler.