Presentation mode shows previous and next slide

[Irubataru]

If I make a presentation in beamer and the aspect ratio of the presentation isn't the same as the monitor it is displayed on, then presentation mode in zathura shows the slide (page) before and after at the top and bottom. I'd rather that it showed black bars. This would also make sure it was centered when showing the first and last slide.

Probably related to #103

How it looks now:

How I expect it to look:

Zathura version:

zathura 0.5.8
girara 0.4.4 (runtime: 0.4.4)
(plugin) ps (0.2.8) (/usr/lib/zathura/libps.so)
(plugin) pdf-mupdf (0.4.4) (/usr/lib/zathura/libpdf-mupdf.so)
(plugin) djvu (0.2.10) (/usr/lib/zathura/libdjvu.so)