3.x: Marbles should have a proper background

The transparent ones look terrible with dark-themed IDEs or non-light background:

Only looks fine in Eclipse:

Write a program that goes through the images in https://github.com/ReactiveX/RxJava.wiki.git images/rx-operators, checks if the image has transparency, draw it onto a white background then save it as originalName.v3.png (because some of those are linked all around). Find the filename in the source code and replace them.

[tomislavhofman]

    private static void makeNonTransparentCopyOfImages(final List<File> files, final File outputFolder) {
        int index = 0;
        for (final File file : files) {
            if (!file.getName().contains(".png")) {
                continue;
            }
            try {
                BufferedImage image = ImageIO.read(file);
                int width = image.getWidth();
                int height = image.getHeight();
                if (image.getColorModel().hasAlpha()) {
                    BufferedImage background = new BufferedImage(width, height, image.getType());
                    Graphics2D g2 = background.createGraphics();
                    g2.setColor(Color.WHITE);
                    g2.fillRect(0, 0, width, height);
                    BufferedImage combined = new BufferedImage(width, height, image.getType());
                    Graphics g = combined.getGraphics();
                    g.drawImage(background, 0, 0, null);
                    g.drawImage(image, 0, 0, null);
                    ImageIO.write(combined, "PNG", new File(outputFolder, file.getName().replace(".png", ".v3.png")));
                } else {
                    System.out.println("Non transparent image: " + index + " - " + file.getName());
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
            index++;
        }
    }

[akarnokd]

@tomislavhofman Sure!