ps: Make sure page labels are valid utf-8 strings

taken from https://git.gnome.org/browse/evince/commit/?id=150d0cfa2d8f64823f2782d207d717fa3ac340a0
create_thumbnail_frame should return NULL when
source_pixbuf is NULL and width and height are not positive.

fix https://bugzilla.redhat.com/show_bug.cgi?id=1082143
taken from https://git.gnome.org/browse/evince/commit/?id=3d8180a605e8c98172e9e0e739e22b7f50ed7bad
djvu: Fix case sensitive search

Closes https://github.com/mate-desktop/atril/issues/41
Closes https://github.com/mate-desktop/atril/pull/74

This commit is based on Aleksi Blinnikka one from #74