v1.0.3: Fix antialiasing problems when drawing partial strokes

When a line segment is appended to the current stroke, we used
to just draw the new line segment on top of the previous picture.
Due to antialiasing, this is gives a different result than drawing
the entire path at the same time: For example, corners of the path
are drawn twice (and therefore twice as dark) with our method.

To solve this issue and maintain reasonable efficiency, we keep two
pictures for each layer: The first picture (A) contains only the strokes
that have been finished. The second picture (B) also contains the stroke
that is currently being drawn. When a line segment is added, we reset
the contents of (B) inside the bounding box of the line segment to the
corresponding contents of (A). We then draw the entire current stroke
on (B), clipped to this bounding box.