Adjust LayerDepthFactor to account for layer size

We need to draw all layers on top of the layer behind us, the factor is
rather arbitrary but needs to ensure that the step is still below 0 when
subtracted using the full layer size

Fixes https://gitlab.com/kicad/code/kicad/issues/19795

pcbnew/router/router_preview_item.h

@@ -73,7 +73,7 @@ public:
      * GAL layer stack into the space of one view group sublayer (which is
      * currently hard-coded via GAL::AdvanceDepth take a depth of 0.1)
      */
-    static constexpr double LayerDepthFactor = 0.0001;
+    static constexpr double LayerDepthFactor = 0.001;
     static constexpr double PathOverlayDepth = LayerDepthFactor * static_cast<double>( LAYER_ZONE_END );
 
     ROUTER_PREVIEW_ITEM( const SHAPE& aShape, PNS::ROUTER_IFACE* aIface, KIGFX::VIEW* aView );