* qt4/src/poppler-link.cc:
        * qt4/src/poppler-page.cc:
        * qt4/src/poppler-qt4.h:
        * qt4/src/poppler-document.cc:
        * qt4/src/poppler-private.h: Generalize the way we render the pages:
        merge all the Page::renderTo* functions in only one that renders on
        a QImage, taking into account the currently chosen backend.
        It is possible to switch rendering backend using the Document.
        Patch by Pino Toscano.

        * qt4/tests/stress-poppler-qt4.cpp:
        * qt4/tests/test-password-qt4.cpp:
        * qt4/tests/test-poppler-qt4.cpp: Adapt the tests to the changes in
        the rendering API of Page. Patch by Pino Toscano.