Fix page boundary gradients on scaled displays

The code that draws a thin gradient at the right and bottom
boundaries of a page implicitly assumed that the display was
not scaled, i.e., that logical pixel where identical to
physical pixels.  Nobody really noticed because that gradient
was so thin.  Anyway, this patch fixes the gradient for
scaled displays.

I considered using a QLinearGradient, but decided against it:
The code would have been longer, and the manual code in the
patch can exploit the fact that all lines are horizontal
or vertical.  Therefore, expensive scan-conversion can be
avoided.

ui/pageview.cpp

@@ -3549,6 +3549,15 @@ void PageView::drawDocumentOnPainter( const QRect & contentsRect, QPainter * p )
     QRect checkRect = contentsRect;
     checkRect.adjust( -3, -3, 1, 1 );
 
+    // Method to linearly interpolate between black (=(0,0,0), omitted) and the background color
+    auto interpolateColor = [&backColor]( double t )
+    {
+        return QColor( t*backColor.red(), t*backColor.green(), t*backColor.blue() );
+    };
+
+    // width of the shadow in device pixels
+    static const int shadowWidth = 2*dpr;
+
     // iterate over all items painting a black outline and a simple bottom/right gradient
     for ( const PageViewItem * item : qAsConst( d->items ) )
     {
@@ -3563,7 +3572,7 @@ void PageView::drawDocumentOnPainter( const QRect & contentsRect, QPainter * p )
         p->save();
         p->translate( itemGeometry.left(), itemGeometry.top() );
 
-        // draw the page outline (black border and 2px bottom-right shadow)
+        // draw the page outline (black border and bottom-right shadow)
         if ( !itemGeometry.contains( contentsRect ) )
         {
             int itemWidth = itemGeometry.width();
@@ -3577,15 +3586,15 @@ void PageView::drawDocumentOnPainter( const QRect & contentsRect, QPainter * p )
             p->drawRect( outline );
 
             // draw bottom/right gradient
-            static const int levels = 2;
-            int r = backColor.red() / (levels + 2) + 6,
-                g = backColor.green() / (levels + 2) + 6,
-                b = backColor.blue() / (levels + 2) + 6;
-            for ( int i = 0; i < levels; i++ )
+            for ( int i = 1; i <= shadowWidth; i++ )
             {
-                p->setPen( QColor( r * (i+2), g * (i+2), b * (i+2) ) );
-                p->drawLine( i, i + itemHeight + 1, i + itemWidth + 1, i + itemHeight + 1 );
-                p->drawLine( i + itemWidth + 1, i, i + itemWidth + 1, i + itemHeight );
+                pen.setColor( interpolateColor( double(i)/( shadowWidth+1 ) ) );
+                p->setPen( pen );
+                QPointF left( (i-1)/dpr, itemHeight + i/dpr );
+                QPointF up( itemWidth + i/dpr, (i-1)/dpr );
+                QPointF corner( itemWidth + i/dpr, itemHeight + i/dpr);
+                p->drawLine( left, corner );
+                p->drawLine( up, corner );
             }
         }