add QgsRectangle::scaled which returns the scaled rectangle

facilitates one-lines

python/core/auto_generated/geometry/qgsrectangle.sip.in

@@ -183,6 +183,13 @@ Scale the rectangle around its center point.
     void scale( double scaleFactor, double centerX, double centerY );
 %Docstring
 Scale the rectangle around its center point.
+%End
+
+    QgsRectangle scaled( double scaleFactor, const QgsPointXY *center = 0 );
+%Docstring
+Scale the rectangle around its ``center`` point.
+
+.. versionadded:: 3.4
 %End
 
     void grow( double delta );

src/core/geometry/qgsrectangle.cpp

@@ -58,6 +58,13 @@ QgsRectangle QgsRectangle::fromCenterAndSize( QgsPointXY center, double width, d
   return QgsRectangle( xMin, yMin, xMax, yMax );
 }
 
+QgsRectangle QgsRectangle::scaled( double scaleFactor, const QgsPointXY *center )
+{
+  QgsRectangle scaledRect = QgsRectangle( *this );
+  scaledRect.scale( scaleFactor, center );
+  return scaledRect;
+}
+
 QgsRectangle QgsRectangle::operator-( const QgsVector v ) const
 {
   double xmin = mXmin - v.x();

src/core/geometry/qgsrectangle.h

@@ -261,6 +261,12 @@ class CORE_EXPORT QgsRectangle
       mYmax = centerY + newHeight / 2.0;
     }
 
+    /**
+     * Scale the rectangle around its \a center point.
+     * \since QGIS 3.4
+     */
+    QgsRectangle scaled( double scaleFactor, const QgsPointXY *center = nullptr );
+
     /**
      * Grows the rectangle in place by the specified amount.
      * \see buffered()