I've reworked the DocText type so it supports fill combinators. Filling pads the right or left of a combinator with space if its length is shorter than the fill - I've only done right fill so far as it is easier.
Also I've added font size changing although this needs careful use as the line height do not adjust dynamically - Line height is judged by the height of the current font when drawing starts, not on the largest height of all subsequent text.
