[pdfx] change scale with PdfControllerPinch

[ogata-k]

Now, cannot change scale with using PdfControllerPinch in pdfx v2.0.1+2. I want to change scale with using PdfControllerPinch.

e.g. (scale to double)

final _pdfController = PdfControllerPinch();
final _currentScale = _pdfController.currentScale;
_pdfController.currentScale = _currentScale * 2;