Toast: lazily create the grid

The toast grid as it is done at the moment uses a lot of memory and
takes a relatively long time to create.

This changes the code so that we only create the grid when a toast
survey needs to be rendered instead of at creation time, saving memory
and startup time in the usual case when we don't use it.

src/core/StelToast.cpp

@@ -220,14 +220,14 @@ void ToastTile::draw(StelPainter* sPainter, const SphericalCap& viewportShape, i
 
 /////// ToastSurvey methods ////////////
 ToastSurvey::ToastSurvey(const QString& path, int amaxLevel)
-	: grid(amaxLevel), path(path), maxLevel(amaxLevel), toastCache(200)
+	: path(path), maxLevel(amaxLevel), toastCache(200)
 {
-	rootTile = new ToastTile(this, 0, 0, 0);
 }
 
 ToastSurvey::~ToastSurvey()
 {
 	delete rootTile;
+	delete grid;
 	rootTile = Q_NULLPTR;
 }
 
@@ -251,6 +251,10 @@ void ToastSurvey::draw(StelPainter* sPainter)
 	const double maxAngle = anglePerPixel * getTilesSize();
 	int maxVisibleLevel = (int)(log2(360. / maxAngle));
 
+	// Lazily creation of the grid and root tile.
+	if (!grid) grid = new ToastGrid(maxLevel);
+	if (!rootTile) rootTile = new ToastTile(this, 0, 0, 0);
+
 	// We also get the viewport shape to discard invisibly tiles.
 	const SphericalCap& viewportRegion = sPainter->getProjector()->getBoundingCap();
 	rootTile->draw(sPainter, viewportRegion, maxVisibleLevel);

src/core/StelToast.hpp

@@ -135,7 +135,7 @@ class ToastSurvey : public QObject
 	virtual ~ToastSurvey();
 	QString getTilePath(int level, int x, int y) const;
 	void draw(StelPainter* sPainter);
-	const ToastGrid* getGrid() const {return &grid;}
+	const ToastGrid* getGrid() const {Q_ASSERT(grid); return grid;}
 	int getMaxLevel() const {return maxLevel;}
 	int getTilesSize() const {return 256;}
 
@@ -146,9 +146,9 @@ class ToastSurvey : public QObject
 	void putIntoCache(ToastTile* tile);
 
 private:
-	ToastGrid grid;
+	ToastGrid* grid = Q_NULLPTR;
 	QString path;
-	ToastTile* rootTile;
+	ToastTile* rootTile = Q_NULLPTR;
 	int maxLevel;
 
 	typedef QCache<ToastTile::Coord, ToastTile> ToastCache;