def centred_in(text, rect)
      size = measure(text)

      Point((rect.width - size.width) / 2, (rect.height - size.height) / 2)
    end
def centred_in(text, rect)
      size = measure(text)

      Point((rect.width - size.width) / 2, (rect.height - size.height) / 2)
    end
Return the co-ordnates needed to place a given string in the centre of an
 area, both vertically and horizontally.

 * +text+    [String] String to centre
 * +rect+    [Size] Rectangular area size

 return::  [Point] The point to write the string, expressed as an offset
           from the top-left corner of the rectangle.