Rectangles might be a little off, but is a lot faster if processing 100s
of rectangles.

This commit fixes attempt to set INFINITY zoom value when opening
document for first time with "adjust-open width" option.

In this case function zathura_document_get_document_size returns 0 width
which leads to incorrect zoom calculation.

Version 0.4.6

https://specifications.freedesktop.org/icon-theme-spec/latest/ar01s07.html

Partially fixes #94