/XYZ destinations: default to top left of the page.

The previous default in the absence of provided coordinates was the
bottom left, so that if you followed a PDF link annotation with a
destination of [page /XYZ null null null] then you would see a gutter
followed by the page _after_ the intended one, because pdf.js had
carefully aligned the lower left corner of the target page with the
top of the window.

As part of this change we allow undefined x,y parameters in URLs with
a &zoom= parameter to get as far as pageViewScrollIntoView instead of
being replaced with zero in pdfViewSetHash, so as to do this
substitution in one place.