renderToPngData
fun renderToPngData(page: PdfPage, scale: Double = 1.0, backgroundR: Double = 1.0, backgroundG: Double = 1.0, backgroundB: Double = 1.0, backgroundA: Double = 1.0): NSData?
Render a page into a PNG-encoded NSData. Returns null if the underlying CGBitmapContext or PNG encoder can't be created (very rare; would indicate a system-level resource failure).