Don't crash when overprint preview is enabled with the Qt bindings
requested to merge ervin/poppler:ervin/dont-crash-when-enabling-overprint-preview-in-qt-bindings into master
When overprint preview is enabled, the image data row size changes during conversion to XBGR. The Qt bindings were assuming this row size was constant which led to badly broken QImages at conversion time (generating garbage if displayed or even crashing).
This commit simply add regression tests for the case and gets the row size after bitmap conversion.