setBackgroundPixmap does not work

[anaselli]

This peace of code (with right path and png) shows that layout background does not work using gtk
`
YDialog * dialog = YUI::widgetFactory()->createPopupDialog(YDialogNormalColor);
YLayoutBox * vbox = YUI::widgetFactory()->createVBox( dialog );
auto bannerMinSize = YUI::widgetFactory()->createMinSize(vbox, 60, 4);
bannerMinSize->setBackgroundPixmap("/home/anaselli/src/manatools/venv/share/icons/hicolor/images/background_dark.png");
YAlignment * align = YUI::widgetFactory()->createAlignment(bannerMinSize, YAlignCenter, YAlignUnchanged);
YImage* img = YUI::widgetFactory()->createImage( align, /home/anaselli/src/manatools/mpan/extras/images/Logo_big_dark.png;

`
QT view:

Gtk one:

[anaselli]

Gtk view after above commit: