Add support for Watermark

[tejpratap46]

Add ability to add watermark on every page of PDF.

[tejpratap46]

Example Code:

@Nullable
@Override
protected PDFImageView getWatermarkView(int forPage) {
    PDFImageView pdfImageView = new PDFImageView(getApplicationContext());
    FrameLayout.LayoutParams childLayoutParams = new FrameLayout.LayoutParams(
            ViewGroup.LayoutParams.MATCH_PARENT,
            200, Gravity.CENTER);
    pdfImageView.setLayout(childLayoutParams);

    pdfImageView.setImageResource(R.drawable.ic_pdf);
    pdfImageView.setImageScale(ImageView.ScaleType.FIT_CENTER);
    pdfImageView.getView().setAlpha(0.3F);

    return pdfImageView;
}