fix(pdfx): center a document shorter than the PdfViewPinch viewport

[ZevyaDev]

When a document is shorter than the viewer, such as a single Letter page on a phone, PdfViewPinch sizes its InteractiveViewer child to the pages, which leaves the child smaller than the viewport. InteractiveViewer won't scale that child below the point where it covers the viewport, so the first pinch jumps to about 1.5× and the page can't be zoomed back out to fit.

#487 worked around this by removing the pan bounds; since #529 that only applies when minScale < 1, so the default viewer has the problem again. This fixes it with the bounds kept: a short document is centered and the laid-out document is always at least the size of the viewport. Zoom now starts at fit and pinching out returns to it. The horizontal layout gets the same treatment.

To reproduce: open a one-page PDF in PdfViewPinch, pinch in, then pinch out.