Minor speed boost to identify highlight

Unfortunately this whole identify highlight class is very inefficient,
resulting in multiple large image redraws at every paint event
and slow iteration over every pixel in these images.

TODO This needs to be completely reworked in future.

src/gui/qgshighlight.cpp

@@ -349,21 +349,23 @@ void QgsHighlight::paint( QPainter *p )
 
       imagePainter.end();
 
-      QColor color( mPen.color() );  // true output color
+      // true output color
+      int penRed = mPen.color().red();
+      int penGreen = mPen.color().green();
+      int penBlue = mPen.color().blue();
       // coefficient to subtract alpha using green (temporary fill)
       double k = ( 255. - mBrush.color().alpha() ) / 255.;
+      QRgb *line = nullptr;
       for ( int r = 0; r < image.height(); r++ )
       {
+        line = ( QRgb * )image.scanLine( r );
         for ( int c = 0; c < image.width(); c++ )
         {
-          QRgb rgba = image.pixel( c, r );
-          int alpha = qAlpha( rgba );
+          int alpha = qAlpha( line[c] );
           if ( alpha > 0 )
           {
-            int green = qGreen( rgba );
-            color.setAlpha( qBound<int>( 0, alpha - ( green * k ), 255 ) );
-
-            image.setPixel( c, r, color.rgba() );
+            int green = qGreen( line[c] );
+            line[c] = qRgba( penRed, penGreen, penBlue, qBound<int>( 0, alpha - ( green * k ), 255 ) );
           }
         }
       }