Fix missing map settings expression variables in composer maps

Missing map extent related variables were causing stacking order
issues with the 25d renderer (fix #14604)

src/core/composer/qgscomposermap.cpp

@@ -2151,6 +2151,14 @@ QgsExpressionContext* QgsComposerMap::createExpressionContext() const
   scope->addVariable( QgsExpressionContextScope::StaticVariable( "map_id", QgsComposerItem::id(), true ) );
   scope->addVariable( QgsExpressionContextScope::StaticVariable( "map_rotation", mMapRotation, true ) );
   scope->addVariable( QgsExpressionContextScope::StaticVariable( "map_scale", scale(), true ) );
+
+  QgsRectangle extent( *currentMapExtent() );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( "map_extent_width", extent.width(), true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( "map_extent_height", extent.height(), true ) );
+  QgsGeometry* centerPoint = QgsGeometry::fromPoint( extent.center() );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( "map_extent_center", QVariant::fromValue( *centerPoint ), true ) );
+  delete centerPoint;
+
   context->appendScope( scope );
 
   return context;

src/core/qgsexpressioncontext.cpp

@@ -713,6 +713,9 @@ void QgsExpressionContextUtils::setLayerVariables( QgsMapLayer* layer, const Qgs
 
 QgsExpressionContextScope* QgsExpressionContextUtils::mapSettingsScope( const QgsMapSettings& mapSettings )
 {
+  // IMPORTANT: ANY CHANGES HERE ALSO NEED TO BE MADE TO QgsComposerMap::createExpressionContext()
+  // (rationale is described in QgsComposerMap::createExpressionContext() )
+
   QgsExpressionContextScope* scope = new QgsExpressionContextScope( QObject::tr( "Map Settings" ) );
 
   //add known map settings context variables