Make QgsRasterIterator optionally return the exact extent of the

current block returned

... because this is very difficult to calculate exactly outside
of the iterator, yet the iterator itself has this knowledge already...

src/core/raster/qgsrasteriterator.cpp

@@ -70,7 +70,7 @@ bool QgsRasterIterator::readNextRasterPart( int bandNumber,
   return result;
 }
 
-bool QgsRasterIterator::readNextRasterPart( int bandNumber, int &nCols, int &nRows, std::unique_ptr<QgsRasterBlock> &block, int &topLeftCol, int &topLeftRow )
+bool QgsRasterIterator::readNextRasterPart( int bandNumber, int &nCols, int &nRows, std::unique_ptr<QgsRasterBlock> &block, int &topLeftCol, int &topLeftRow, QgsRectangle *blockExtent )
 {
   QgsDebugMsgLevel( QStringLiteral( "Entered" ), 4 );
   block.reset();
@@ -112,6 +112,9 @@ bool QgsRasterIterator::readNextRasterPart( int bandNumber, int &nCols, int &nRo
   double ymax = viewPortExtent.yMaximum() - pInfo.currentRow / static_cast< double >( pInfo.nRows ) * viewPortExtent.height();
   QgsRectangle blockRect( xmin, ymin, xmax, ymax );
 
+  if ( blockExtent )
+    *blockExtent = blockRect;
+
   block.reset( mInput->block( bandNumber, blockRect, nCols, nRows, mFeedback ) );
   topLeftCol = pInfo.currentCol;
   topLeftRow = pInfo.currentRow;

src/core/raster/qgsrasteriterator.h

@@ -70,14 +70,16 @@ class CORE_EXPORT QgsRasterIterator
      * \param block address of block pointer
      * \param topLeftCol top left column
      * \param topLeftRow top left row
+     * \param blockExtent optional storage for exact extent of returned raster block
      * \returns false if the last part was already returned
      * \note Not available in Python bindings
      * \since QGIS 3.2
     */
     bool readNextRasterPart( int bandNumber,
                              int &nCols, int &nRows,
                              std::unique_ptr< QgsRasterBlock > &block,
-                             int &topLeftCol, int &topLeftRow ) SIP_SKIP;
+                             int &topLeftCol, int &topLeftRow,
+                             QgsRectangle *blockExtent = nullptr ) SIP_SKIP;
 
     void stopRasterRead( int bandNumber );