Only emit cameraChanged() on frame start

Previously the entire camera change was handled on every input event,
which led to slowdown and stutters, since we can get ~1000/s such events
with common mice. We don't care about intermediate camera states and
can only process the last state before rendering.

src/3d/qgscameracontroller.cpp

@@ -161,6 +161,9 @@ void QgsCameraController::zoomCameraAroundPivot( const QVector3D &oldCameraPosit
 void QgsCameraController::frameTriggered( float dt )
 {
   Q_UNUSED( dt )
+
+  if ( mCameraChanged )
+    emit cameraChanged();
 }
 
 void QgsCameraController::resetView( float distance )
@@ -282,7 +285,7 @@ void QgsCameraController::updateCameraFromPose()
   if ( mCamera )
     mCameraPose.updateCamera( mCamera );
 
-  emit cameraChanged();
+  mCameraChanged = true;
 }
 
 void QgsCameraController::moveCameraPositionBy( const QVector3D &posDiff )

src/3d/qgscameracontroller.h

@@ -385,6 +385,9 @@ class _3D_EXPORT QgsCameraController : public QObject
     // 3D world's origin in map coordinates
     QgsVector3D mOrigin;
 
+    //! Did camera change since last frame? Need to know if we should emit cameraChanged().
+    bool mCameraChanged = false;
+
     // To test the cameracontroller
     friend class TestQgs3DRendering;
     friend class TestQgs3DCameraController;