Use Graphics2D for fun and profit

Plugins now take a Graphics2D as an argument, and draw on it directly.
This has several advantages:
— Cleaner code: result.add(new Instruction(Instruction.Kind.LINE), x, y)
    becomes simply g.lineTo(x, y)
— Free work: Graphics2D handles most of the complex stuff about vectorial
    graphics (segmenting Bézier curves, clipping paths, affine transforms…)
— Flexibility: the application is no longer limited to outputting text;
    by passing appropriate Graphics2D instances to a plugin, it is now
    possible to draw the vectors on the screen or physically print them,
    without any change to the plugin code