Better display of incomplete multipolygons

[stoecker]

Incomplete multipolygons may be displayed a lot better, when they are clipped at the download boundaries. It is not possible to get this always right, but in most cases a good guess should be usable to improve display a lot.

[bastiK]

Here is some simple clipping (by Gubaer):

​https://github.com/openstreetmap/josm/compare/ca3cee8be00739941a1527aef84fa0496990fc6b...327561e86fed93965a5a#diff-9

    /**
     * <p>Prepares the clipping area of the graphics context in such a way, that no
     * points, lines or areas are rendered outside the data source bounding boxes in
     * {@code ds}.</p>
     * 
     * @param ds the data set. If null, no clipping area is set.
     */
    public void prepareClippingAgainstDownloadArea(DataSet ds) {
        if (ds == null) return;
        Area clip = null;
        for (Bounds bounds: ds.getDataSourceBounds()) {
            if (bounds.isCollapsed()) {
                continue;
            }
            Point p1 = nc.getPoint(bounds.getMin());
            Point p2 = nc.getPoint(bounds.getMax());
            Rectangle r = new Rectangle(Math.min(p1.x, p2.x),Math.min(p1.y, p2.y),Math.abs(p2.x-p1.x),Math.abs(p2.y-p1.y));
            if (clip == null){
                clip = new Area(r);
            } else {
                clip.add(new Area(r));
            }
        }
        if (clip != null && !clip.isEmpty()) {
            /*
             * Clipping area of g at this point already set to the map viewport. Intersect with
             * the areas of bounding boxes.
             */
            Shape formerClip = g.getClip();
            if (formerClip != null){
                clip.intersect(new Area(formerClip));
            }
        }

        if (clip != null && !clip.isEmpty()) {
            g.setClip(clip);
        }
    }

[Don-vip]

See also #6020 and #7635.