Set VTK DPI in the render window set in QVTKOpenGLWidget

This improves chart font size (and ideally other chart element size)
on HiDPI displays.

GUISupport/Qt/QVTKOpenGLWidget.cxx

@@ -377,6 +377,7 @@ void QVTKOpenGLWidget::requireRenderWindowInitialization()
   if (this->RenderWindow)
   {
     this->RenderWindow->SetReadyForRendering(false);
+    this->markCachedImageAsDirty();
   }
 }
 
@@ -428,6 +429,10 @@ void QVTKOpenGLWidget::paintGL()
     f->glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_SAMPLES, &samples);
     this->RenderWindow->SetMultiSamples(static_cast<int>(samples));
 
+    // Set the render window's DPI so that graphical elements in charts, etc.
+    // are not super tiny.
+    this->RenderWindow->SetDPI(this->devicePixelRatio() * this->physicalDpiX());
+
     // On OsX (if QSurfaceFormat::alphaBufferSize() > 0) or when using Mesa, we
     // end up rendering fully transparent windows (see through background)
     // unless we fill it with alpha=1.0 (See paraview/paraview#17159).