(bug 37692) Enforce one-based page numbers

ImageMagick identify >= 6.6.8-10 returns zero-based page numbers, which
breaks this extension and leads to an error "The uploaded file contains
errors: inconsistent page numbering in TIFF directory".

This change maps the page numbers as returned by identify to a one-based
sequence.

Change-Id: I25052af7adb6f116c3f6a06652b5c0ce92c9d3a5
Merge "(bug 37692) Enforce one-based page numbers"