Expose device-pixel-ratio to properties.

This allows scaling of the UI accordingly, to keep things usable on
HiDPi screens. (Note this is not fixed yet, but this is a step)

src/Viewer/GraphicsWindowQt5.cpp

@@ -141,6 +141,12 @@ void GLWindow::onScreenChanged()
 #if (QT_VERSION >= QT_VERSION_CHECK(5, 2, 0))
     _devicePixelRatio = screen()->devicePixelRatio();
 #endif
+    
+    if (_isPrimaryWindow) {
+        // allow PUI and Canvas to be scaled
+        fgSetDouble("/sim/rendering/gui-pixel-ratio", _devicePixelRatio);
+    }
+    
     syncGeometryWithOSG();
 }
 
@@ -355,8 +361,6 @@ void GLWindow::wheelEvent( QWheelEvent* event )
 }
 
 GraphicsWindowQt5::GraphicsWindowQt5(osg::GraphicsContext::Traits* traits)
-:   _realized(false)
-,   _updateContextNeeded(false)
 {
     _traits = traits;
     init(0);
@@ -385,14 +389,12 @@ bool GraphicsWindowQt5::init( Qt::WindowFlags f )
         //flags |= Qt::MaximizeUsingFullscreenGeometryHint;
     }
     
-    // create widget
+    // create window
     _window.reset(new GLWindow);
     _window->setFlags(flags);
     _window->setSurfaceType(QSurface::OpenGLSurface);
     _window->setFormat(traits2qSurfaceFormat(_traits.get()));
     _window->create();
-
-
     _window->setTitle( _traits->windowName.c_str() );
     
     // to get OS-dependant default positioning of the window (which is desirable),
@@ -418,6 +420,10 @@ bool GraphicsWindowQt5::init( Qt::WindowFlags f )
     }
 
     _window->_isPrimaryWindow = windowData->isPrimaryWindow;
+    if (_window->_isPrimaryWindow) {
+        fgSetDouble("/sim/rendering/gui-pixel-ratio", _window->_devicePixelRatio);
+    }
+    
     _window->setGraphicsWindow( this );
     useCursor( _traits->useCursor );
 
@@ -760,7 +766,11 @@ void GraphicsWindowQt5::requestRedraw()
 
 void GraphicsWindowQt5::requestContinuousUpdate(bool needed)
 {
+    _continousUpdate = needed;
     GraphicsWindow::requestContinuousUpdate(needed);
+    if (_continousUpdate) {
+        _window->requestUpdate();
+    }
 }
 
 void GraphicsWindowQt5::setFullscreen(bool isFullscreen)

src/Viewer/GraphicsWindowQt5.hxx

@@ -112,7 +112,7 @@ protected:
     QSet<QEvent::Type> _eventCompressor;
 
     bool _forwardKeyEvents = false;
-    qreal _devicePixelRatio;
+    qreal _devicePixelRatio = 1.0;
     
     // is this the primary (GUI) window
     bool _isPrimaryWindow = false;
@@ -198,8 +198,9 @@ protected:
     QOpenGLContext* _shareContext = nullptr;
     bool _ownsWidget;
     QCursor _currentCursor;
-    bool _realized;
-    bool _updateContextNeeded;
+    bool _realized = false;
+    bool _updateContextNeeded = false;
+    bool _continousUpdate = false;
     osg::observer_ptr< osgViewer::ViewerBase > _viewer;
     
     // if true, we will generate a resize event on the next

src/Viewer/fg_os_osgviewer.cxx

@@ -372,6 +372,8 @@ void fgOSInit(int* argc, char** argv)
         SG_LOG(SG_GL, SG_INFO, "Using Qt implementation of GraphicsWindow");
         flightgear::initQtWindowingSystem();
     } else {
+        // stock OSG windows are not Hi-DPI aware
+        fgSetDouble("/sim/rendering/gui-pixel-ratio", 1.0);
         SG_LOG(SG_GL, SG_INFO, "Using stock OSG implementation of GraphicsWindow");
     }
 #endif