Be more careful with alpha value.

Fixes https://gitlab.com/kicad/code/kicad/issues/8850

pcbnew/pcb_painter.cpp

@@ -1709,7 +1709,7 @@ void PCB_PAINTER::draw( const ZONE* aZone, int aLayer )
 
     if( m_pcbSettings.m_zoneOutlines && outline && outline->OutlineCount() > 0 )
     {
-        m_gal->SetStrokeColor( color.WithAlpha( 1.0 ) );
+        m_gal->SetStrokeColor( color.a > 0.0 ? color.WithAlpha( 1.0 ) : color );
         m_gal->SetIsFill( false );
         m_gal->SetIsStroke( true );
         m_gal->SetLineWidth( m_pcbSettings.m_outlineWidth );