Delay main view update until the next frame

This improves a lot the responsiveness of mac touchpad.  Still not as
smooth as with a mouse though, but almost good enough (on my Macbook
air).

I am not totally sure why this works to be honest, but it's not too bad
to use a singleShot(0, ...) call since it clearly shows the intent
that is to render as soon as possible after the current frame.

src/StelMainView.cpp

@@ -1297,7 +1297,7 @@ void StelMainView::drawEnded()
 	{
 		updateQueued = true;
 		minFpsTimer->stop();
-		glWidget->update();
+		QTimer::singleShot(0, glWidget, SLOT(update()));
 	}
 	else
 	{
@@ -1328,7 +1328,7 @@ void StelMainView::minFPSUpdate()
 	{
 		updateQueued = true;
 		//qDebug()<<"minFPSUpdate";
-		glWidget->update();
+		QTimer::singleShot(0, glWidget, SLOT(update()));
 	}
 	else
 	{