fixed disappering tiles on zoom 1-3

src/Tiles/Projections/BaseProjection.h

@@ -91,7 +91,7 @@ class BaseProjection
 	void GetTileMatrixSizeXY(int zoom, CSize &ret);
 	void getTileRectXY(Extent extentsWgs84, int zoom, CRect &rect);
 
-	RectLatLng CalculateGeogBounds(CPoint pnt, int zoom);
+	virtual RectLatLng CalculateGeogBounds(CPoint pnt, int zoom);
 
 	void Clip(CPoint& tilePnt, int zoom);
 

src/Tiles/Projections/MercatorProjection.cpp

@@ -117,3 +117,35 @@ void MercatorProjection::FromXYToLatLng(CPoint pnt, int zoom, PointLatLng &ret)
 	ret.Lng = 360 * xx;
 }
 
+// ****************************************************
+//		CalculateGeogBounds
+// ****************************************************
+// check result from BaseProjection against min/max lat/lon, because BaseProjection don't 
+// have lat/lon limits, but Mercator do
+RectLatLng MercatorProjection::CalculateGeogBounds(CPoint pnt, int zoom)
+{
+	auto res = BaseProjection::CalculateGeogBounds(pnt, zoom);
+
+	//sanity check
+
+	if (res.yLat > _maxLat)
+		res.yLat = _maxLat;
+
+	if (res.yLat < _minLat)
+		res.yLat = _minLat;
+
+	if (res.xLng > _maxLng)
+		res.xLng = _maxLng;
+
+	if (res.xLng < _minLng)
+		res.xLng = _minLng;
+
+	if ((res.xLng + res.WidthLng) > _maxLng)
+		res.WidthLng = _maxLng - res.xLng;
+
+	if ((res.yLat - res.HeightLat) < _minLat)
+		res.HeightLat = res.yLat - _minLat;
+
+	return res;
+}
+

src/Tiles/Projections/MercatorProjection.h

@@ -60,4 +60,6 @@ class MercatorProjection: public BaseProjection
 	// converts tile coordinates to decimal degrees
 	void FromXYToLatLng(CPoint pnt, int zoom, PointLatLng &ret);
 
+	// override BaseProjection's member to check result
+	virtual RectLatLng CalculateGeogBounds(CPoint pnt, int zoom);
 };