Do not persist estimated GDAL metadata

GDAL saves metadata like min and max values into a .aux.xml sidecar file next to raster files.
It does this always, even when the calculated values are estimated. In subsequent runs of GDAL processing tools
it will use these values as if they were reliable.

This patch takes care of deleting newly written .aux.xml files if there is a risk that they include estimated data.

Fix #19517  https://issues.qgis.org/issues/19517

src/providers/gdal/qgsgdalprovider.cpp

@@ -470,7 +470,15 @@ QgsGdalProvider::~QgsGdalProvider()
       }
       if ( mGdalDataset )
       {
+        // Check if already a PAM (persistent auxiliary metadata) file exists
+        QString pamFile = dataSourceUri( true ) + QLatin1String( ".aux.xml" );
+        bool pamFileAlreadyExists = QFileInfo( pamFile ).exists();
+
         GDALClose( mGdalDataset );
+
+        // If GDAL created a PAM file right now by using estimated metadata, delete it right away
+        if ( !mStatisticsAreReliable && !pamFileAlreadyExists && QFileInfo( pamFile ).exists() )
+          QFile( pamFile ).remove();
       }
 
       if ( mpParent && *mpParent == this )
@@ -2468,6 +2476,7 @@ QgsRasterBandStats QgsGdalProvider::bandStatistics( int bandNo, int stats, const
     myerval = GDALComputeRasterStatistics( myGdalBand, bApproxOK,
                                            &pdfMin, &pdfMax, &pdfMean, &pdfStdDev,
                                            progressCallback, &myProg );
+    mStatisticsAreReliable = true;
   }
   else
   {

src/providers/gdal/qgsgdalprovider.h

@@ -296,6 +296,8 @@ class QgsGdalProvider : public QgsRasterDataProvider, QgsGdalProviderBase
      * Converts a world (\a x, \a y) coordinate to a pixel \a row and \a col.
      */
     bool worldToPixel( double x, double y, int &col, int &row ) const;
+
+    bool mStatisticsAreReliable = false;
 };
 
 #endif