Fix return of buffer for PNG QR image generation - resolves #2124 (#2125)

Co-authored-by: GCHQ Developer 85297 <95289555+C85297@users.noreply.github.com>

src/core/lib/QRCode.mjs

@@ -106,7 +106,7 @@ export function generateQrCode(
         case "PDF":
             return Utils.strToArrayBuffer(qrImage);
         case "PNG":
-            return qrImage.buffer;
+            return qrImage.buffer.slice(qrImage.byteOffset, qrImage.byteLength + qrImage.byteOffset);
         default:
             throw new OperationError("Unsupported QR code format.");
     }