Library: eliminate QPainter warnings

There were many run-time warnings in YACReaderLibrary built in Debug
mode with hardware acceleration disabled and with ClassicComicsView
as the active comics view:
    QWidget::paintEngine: Should no longer be called
    QPainter::begin: Paint device returned engine == 0, type: 1

The ComicFlowWidgetSW::paintEvent() implementation now corresponds to
ComicFlowWidgetGL::paintEvent(), which has been fixed earlier.

QWidget::repaint() calls paintEvent() immediately.
PictureFlow::paintEvent() calls d->renderer->paint(). So the
d->renderer->paint() call in PictureFlow::updateMarks() was redundant.

YACReaderLibrary/comic_flow_widget.cpp

@@ -105,7 +105,7 @@ void ComicFlowWidgetSW::keyPressEvent(QKeyEvent* event)
 }
 void ComicFlowWidgetSW::paintEvent(QPaintEvent *event)
 {
-	flow->paintEvent(event);
+	ComicFlowWidget::paintEvent(event);
 }
 void ComicFlowWidgetSW::mousePressEvent(QMouseEvent* event)
 {