visualisations: Properly scale projectM view

In version 5.6, Qt introduced an automatic scaling feature for high DPI
displays. Since projectM is not part of the Qt framework, it's necessary
to convert coordinates when specifying view size.

Reference: https://doc.qt.io/qt-5/highdpi.html

src/visualisations/projectmvisualisation.cpp

@@ -48,12 +48,14 @@
 #include <GL/gl.h>
 #endif
 
-ProjectMVisualisation::ProjectMVisualisation(QObject* parent)
-    : QGraphicsScene(parent),
+ProjectMVisualisation::ProjectMVisualisation(VisualisationContainer* container)
+    : QGraphicsScene(container),
       preset_model_(nullptr),
       mode_(Random),
       duration_(15),
-      texture_size_(512) {
+      texture_size_(512),
+      pixel_ratio_(container->devicePixelRatio()),
+      container_(container) {
   connect(this, SIGNAL(sceneRectChanged(QRectF)),
           SLOT(SceneRectChanged(QRectF)));
 
@@ -143,14 +145,21 @@ void ProjectMVisualisation::drawBackground(QPainter* p, const QRectF&) {
     InitProjectM();
   }
 
-  projectm_->projectM_resetGL(sceneRect().width(), sceneRect().height());
+  projectm_->projectM_resetGL(sceneRect().width() * pixel_ratio_,
+                              sceneRect().height() * pixel_ratio_);
   projectm_->renderFrame();
 
   p->endNativePainting();
 }
 
 void ProjectMVisualisation::SceneRectChanged(const QRectF& rect) {
-  if (projectm_) projectm_->projectM_resetGL(rect.width(), rect.height());
+  // NOTE: This should be updated on a QScreen dpi change signal. Accessing the
+  // QScreen becomes a lot easier in Qt 5.14 with QWidget::screen().
+  pixel_ratio_ = container_->devicePixelRatio();
+
+  if (projectm_)
+    projectm_->projectM_resetGL(rect.width() * pixel_ratio_,
+                                rect.height() * pixel_ratio_);
 }
 
 void ProjectMVisualisation::SetTextureSize(int size) {

src/visualisations/projectmvisualisation.h

@@ -29,12 +29,14 @@ class projectM;
 
 class ProjectMPresetModel;
 
+class VisualisationContainer;
+
 class QTemporaryFile;
 
 class ProjectMVisualisation : public QGraphicsScene, public BufferConsumer {
   Q_OBJECT
  public:
-  ProjectMVisualisation(QObject* parent = nullptr);
+  ProjectMVisualisation(VisualisationContainer* container);
   ~ProjectMVisualisation();
 
   enum Mode {
@@ -87,6 +89,12 @@ class ProjectMVisualisation : public QGraphicsScene, public BufferConsumer {
   std::vector<int> default_rating_list_;
 
   int texture_size_;
+  // As of version 5.6, Qt supports automatic scaling for high-DPI displays. We
+  // need to know the pixel ratio so that we can convert coordinates for
+  // projectM.
+  // https://doc.qt.io/qt-5/highdpi.html
+  qreal pixel_ratio_;
+  VisualisationContainer* container_;
 };
 
 #endif  // PROJECTMVISUALISATION_H