Simplify `QPainter::drawRoundedRect` usage

After fixing the porting mistake done in 78d983ee, `drawRoundedRect` can
be further simplified to not use any relative sizes anymore.

We can also assume that the code in `dviRenderer::epsf_special` was
supposed to use a constant corner radius and therefore was wrong
initially. By dropping `Qt::RelativeSize` the code might become closer
to its originally intended behavior.

Test Plan:
  - No change in behavior for rounded corners of "Welcome" popup message
    and "No Annotations" message in sidebar.
  - Could not test DVI code path.

generators/dvi/special.cpp

@@ -433,7 +433,7 @@ void dviRenderer::epsf_special(const QString &cp)
             foreGroundPainter->setBrush(Qt::red);
         }
         foreGroundPainter->setPen(Qt::black);
-        foreGroundPainter->drawRoundedRect(bbox, 2, 2, Qt::RelativeSize);
+        foreGroundPainter->drawRoundedRect(bbox, 2, 2);
         QFont f = foreGroundPainter->font();
         f.setPointSize(8);
         foreGroundPainter->setFont(f);

part/pageviewutils.cpp

@@ -360,7 +360,7 @@ void PageViewMessage::paintEvent(QPaintEvent * /* e */)
     painter.setPen(Qt::black);
     painter.setBrush(palette().color(QPalette::Window));
     painter.translate(0.5, 0.5);
-    painter.drawRoundedRect(1, 1, width() - 2, height() - 2, 600.0 / width(), 600.0 / height(), Qt::RelativeSize);
+    painter.drawRoundedRect(1, 1, width() - 2, height() - 2, 3, 3);
 
     // draw icon if present
     if (!m_symbol.isNull()) {

part/side_reviews.cpp

@@ -72,7 +72,7 @@ protected:
 
             p.setBrush(palette().window());
             p.translate(0.5, 0.5);
-            p.drawRoundedRect(15, 15, w, h, (3 * 200.0) / w, (3 * 200.0) / h, Qt::RelativeSize);
+            p.drawRoundedRect(15, 15, w, h, 3, 3);
             p.translate(20, 20);
             document.drawContents(&p);