TrackView: do not set Qt::WA_OpaquePaintEvent attribute

Due to optimizations in Qt 4.5 setting Qt::WA_OpaquePaintEvent attribute
for TrackView widgets caused a line of distorted pixels at the bottom of
the left part so do not set this attribute anymore (closes #2795861).
(cherry picked from commit cdcc158c03)

src/core/track.cpp

@@ -2107,7 +2107,6 @@ trackView::trackView( track * _track, trackContainerView * _tcv ) :
 
 	setAcceptDrops( true );
 	setAttribute( Qt::WA_DeleteOnClose, true );
-	setAttribute( Qt::WA_OpaquePaintEvent, true );
 
 
 	connect( m_track, SIGNAL( destroyedTrack() ), this, SLOT( close() ) );