Fix missing initialization (#7271)

Fix the missing initialization of some variables in `TrackContentWidget`.
This led to some performances issues when the widget was painted because
a for loop was executed for which the variable started at a very large
negative number and was then incremented.

src/gui/tracks/TrackContentWidget.cpp

@@ -70,7 +70,12 @@ TrackContentWidget::TrackContentWidget( TrackView * parent ) :
 	m_coarseGridColor( Qt::SolidPattern ),
 	m_fineGridColor( Qt::SolidPattern ),
 	m_horizontalColor( Qt::SolidPattern ),
-	m_embossColor( Qt::SolidPattern )
+	m_embossColor( Qt::SolidPattern ),
+	m_coarseGridWidth(2),
+	m_fineGridWidth(1),
+	m_horizontalWidth(1),
+	m_embossWidth(0),
+	m_embossOffset(0)
 {
 	setAcceptDrops( true );