Restore mouse drag scrolling, so it starts immediately

QScroller uses a minimum drag distance before drag scrolling starts by default.
This sets the minimum distance to 0.0, to restore the original behavior of the Browse tool.

BUG: 420842

ui/pageview.cpp

@@ -421,6 +421,7 @@ PageView::PageView(QWidget *parent, Okular::Document *document)
     prop.setScrollMetric(QScrollerProperties::MaximumVelocity, 1);
     prop.setScrollMetric(QScrollerProperties::HorizontalOvershootPolicy, QScrollerProperties::OvershootAlwaysOff);
     prop.setScrollMetric(QScrollerProperties::VerticalOvershootPolicy, QScrollerProperties::OvershootAlwaysOff);
+    prop.setScrollMetric(QScrollerProperties::DragStartDistance, 0.0);
     d->scroller->setScrollerProperties(prop);
 
     connect(d->scroller, &QScroller::stateChanged, this, &PageView::slotRequestVisiblePixmaps);