JAL-3444 SwingJS-site.zip fix and renderer.seqfeatures fixes

JAL-3444 SwingJS-site.zip fix and renderer.seqfeatures fixes

- BufferedImage implements Image.flush() to clear pixel cache;
- JSGraphics2D check for opacity 1 before clearing HTML5 canvas
AlphaComposite

- FeatureColourFinder applies

ifJS() {
        g.setColor(Color.white);
        g.fillRect(0, 0, 1, 1);
}

so as to clear the HTML5 canvas image before applying any other color.
Also applies

    if (Platform.isJS())
    {
      offscreenImage.flush();
    }

just before offscreenImage.getRGB(0,0) to force the image to recreate
its cached pixel from the HTML5 canvas, which is drawn to directly.