avoid scaling in painting when possible

not sure how it works internally in Qt, but no need to call epxlicitely on scaling since the image device pixel ratio is set

src/gui/qgsmapcanvasmap.cpp

@@ -60,12 +60,20 @@ QRectF QgsMapCanvasMap::boundingRect() const
 
 void QgsMapCanvasMap::paint( QPainter *painter )
 {
-  int w = std::round( mItemSize.width() ) - 2, h = std::round( mItemSize.height() ) - 2; // setRect() makes the size +2 :-(
-  if ( mImage.size() != QSize( w, h ) )
+  // setRect() makes the size +2 :-(
+  int w = std::round( mItemSize.width() ) - 2;
+  int h = std::round( mItemSize.height() ) - 2;
+
+  bool scale = false;
+  if ( mImage.size() != QSize( w, h )*mImage.devicePixelRatioF() )
   {
-    QgsDebugMsg( QStringLiteral( "map paint DIFFERENT SIZE: img %1,%2  item %3,%4" ).arg( mImage.width() ).arg( mImage.height() ).arg( w ).arg( h ) );
+    QgsDebugMsg( QStringLiteral( "map paint DIFFERENT SIZE: img %1,%2  item %3,%4" )
+                 .arg( mImage.width() / mImage.devicePixelRatioF() )
+                 .arg( mImage.height() / mImage.devicePixelRatioF() )
+                 .arg( w ).arg( h ) );
     // This happens on zoom events when ::paint is called before
     // the renderer has completed
+    scale = true;
   }
 
   /*Offset between 0/0 and mRect.xMinimum/mRect.yMinimum.
@@ -83,12 +91,10 @@ void QgsMapCanvasMap::paint( QPainter *painter )
     painter->drawImage( QRectF( ul.x(), ul.y(), lr.x() - ul.x(), lr.y() - ul.y() ), imIt->first, QRect( 0, 0, imIt->first.width(), imIt->first.height() ) );
   }
 
-  qDebug() << "map painter: " << w << h << mImage.size() << mImage.devicePixelRatioF();
-  painter->drawImage( QRect( 0, 0, w, h ), mImage );
-  //Q_ASSERT(mImage.isNull() || mImage.devicePixelRatio()==2);
-  //QImage img = mImage;
-  //img.setDevicePixelRatio(2);
-  //painter->drawImage( 0,0, img );
+  if ( scale )
+    painter->drawImage( QRect( 0, 0, w, h ), mImage );
+  else
+    painter->drawImage( 0, 0, mImage );
 
   // For debugging:
 #if 0