Base class implementation of sample uses identify

This allows derived classes with optimised identify methods to
utilise the same optimisations for sample. Better would be for
them to implement optimised sample methods, but this is better
than using the unoptimised block approach for all providers.

src/core/raster/qgsrasterdataprovider.cpp

@@ -301,48 +301,21 @@ QgsRasterIdentifyResult QgsRasterDataProvider::identify( const QgsPointXY &point
 }
 
 double QgsRasterDataProvider::sample( const QgsPointXY &point, int band,
-                                      bool *ok, const QgsRectangle &boundingBox, int width, int height, int )
+                                      bool *ok, const QgsRectangle &boundingBox, int width, int height, int dpi )
 {
   if ( ok )
     *ok = false;
 
-  if ( !extent().contains( point ) )
-  {
-    // Outside the raster
-    return std::numeric_limits<double>::quiet_NaN();
-  }
-
-  QgsRectangle finalExtent = boundingBox;
-  if ( finalExtent.isEmpty() )
-    finalExtent = extent();
-
-  if ( width == 0 )
-  {
-    width = capabilities() & Size ? xSize() : 1000;
-  }
-  if ( height == 0 )
-  {
-    height = capabilities() & Size ? ySize() : 1000;
-  }
-
-  // Calculate the row / column where the point falls
-  double xres = ( finalExtent.width() ) / width;
-  double yres = ( finalExtent.height() ) / height;
-
-  int col = static_cast< int >( std::floor( ( point.x() - finalExtent.xMinimum() ) / xres ) );
-  int row = static_cast< int >( std::floor( ( finalExtent.yMaximum() - point.y() ) / yres ) );
+  const auto res = identify( point, QgsRaster::IdentifyFormatValue, boundingBox, width, height, dpi );
+  const QVariant value = res.results().value( band );
 
-  double xMin = finalExtent.xMinimum() + col * xres;
-  double xMax = xMin + xres;
-  double yMax = finalExtent.yMaximum() - row * yres;
-  double yMin = yMax - yres;
-  QgsRectangle pixelExtent( xMin, yMin, xMax, yMax );
+  if ( !value.isValid() )
+    return std::numeric_limits<double>::quiet_NaN();
 
-  std::unique_ptr< QgsRasterBlock > bandBlock( block( band, pixelExtent, 1, 1 ) );
-  if ( bandBlock && ok )
+  if ( ok )
     *ok = true;
 
-  return bandBlock ? bandBlock->value( 0 ) : std::numeric_limits<double>::quiet_NaN();
+  return value.toDouble( ok );
 }
 
 QString QgsRasterDataProvider::lastErrorFormat()