Fix High DPI for canvas previews in Undo History docker

Before this commit, canvas previews/thumbnails in the
Undo History docker were low resolution on High DPI screen.
This commit make them high resolution (respect devicePixelRatio).
Because there is no item delegate and no manual paint command,
it required adding a devicePixelRatio parameter to the Model.

plugins/dockers/historydocker/KisUndoModel.cpp

@@ -255,9 +255,11 @@ void KisUndoModel::addImage(int idx)
     if (m_stack->count() == idx && !m_imageMap.contains(currentCommand)) {
         KisImageWSP historyImage = m_canvas->image();
         KisPaintDeviceSP paintDevice = historyImage->projection();
-        QImage image = paintDevice->createThumbnail(32, 32, 1,
+        QSize size = QSize(32, 32)*m_devicePixelRatioF;
+        QImage image = paintDevice->createThumbnail(size.width(), size.height(), 1,
                                                     KoColorConversionTransformation::internalRenderingIntent(),
                                                     KoColorConversionTransformation::internalConversionFlags());
+        image.setDevicePixelRatio(m_devicePixelRatioF);
         m_imageMap[currentCommand] = image;
     }
 
@@ -281,3 +283,8 @@ bool KisUndoModel::checkMergedCommand(int index)
     Q_UNUSED(index)
     return false;
 }
+
+void KisUndoModel::setDevicePixelRatio(qreal devicePixelRatio)
+{
+    m_devicePixelRatioF = devicePixelRatio;
+}

plugins/dockers/historydocker/KisUndoModel.h

@@ -93,6 +93,14 @@ public:
     void setCanvas(KisCanvas2* canvas);
     bool checkMergedCommand(int index);
 
+    // It's a bit weird to have "setDevicePixelRatio"
+    //  in a model class, but it's needed for
+    //  QImages inside, because there is no
+    //  ItemDelegate for the items,
+    //  so it's better to initialize QImages already
+    //  devicePixelRatio-corrected.
+    void setDevicePixelRatio(qreal devicePixelRatio);
+
 public Q_SLOTS:
     void setStack(KUndo2QStack *stack);
     void addImage(int idx);
@@ -110,5 +118,6 @@ private:
     QIcon m_clean_icon;
     QPointer<KisCanvas2> m_canvas;
     QMap<const KUndo2Command*, QImage> m_imageMap;
+    qreal m_devicePixelRatioF {1.0};
 };
 #endif

plugins/dockers/historydocker/KisUndoView.cpp

@@ -118,6 +118,7 @@ void KisUndoViewPrivate::init(KisUndoView* view)
 {
     q = view;
     model = new KisUndoModel(q);
+    model->setDevicePixelRatio(view->devicePixelRatioF());
     q->setModel(model);
     q->setSelectionModel(model->selectionModel());
 }