pcbnew: Make clearance translucent

This sets the clearance outline to a slightly better opacity to allow
viewing of items behind the routed track clearance.  Future work should
include making this a configuration option.

Fixes: lp:1826719
* https://bugs.launchpad.net/kicad/+bug/1826719

(cherry picked from commit e0589d2908)

pcbnew/router/router_preview_item.cpp

@@ -173,8 +173,9 @@ void ROUTER_PREVIEW_ITEM::ViewDraw( int aLayer, KIGFX::VIEW* aView ) const
         // Cairo doesn't current support z-ordering, so we need
         // to draw the clearance first to ensure it is in the background
         gal->SetLayerDepth( ClearanceOverlayDepth );
-        gal->SetStrokeColor( COLOR4D( DARKDARKGRAY ) );
-        gal->SetFillColor( COLOR4D( DARKDARKGRAY ) );
+        //TODO(snh) Add configuration option for the color/alpha here
+        gal->SetStrokeColor( COLOR4D( DARKDARKGRAY ).WithAlpha( 0.9 ) );
+        gal->SetFillColor( COLOR4D( DARKDARKGRAY ).WithAlpha( 0.7 ) );
         gal->SetIsStroke( m_width ? true : false );
         gal->SetIsFill( true );