encodeToPng
Encode this ImageBitmap to PNG bytes — the "saveable image" counterpart of PdfView's onPageRendered callback. Write the result to a file, share it, upload it; PNG is lossless so the export is pixel-identical to what was on screen.
Return
PNG bytes, or null if the platform failed to encode (corrupt or zero-sized bitmap — not expected for bitmaps produced by PdfView).