Add "Mechanical" pad property

Prevents mechanical support pads from being used to check pad types.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/16747

pcbnew/plot_brditems_plotter.cpp

@@ -184,6 +184,7 @@ void BRDITEMS_PLOTTER::PlotPad( const PAD* aPad, const COLOR4D& aColor, OUTLINE_
             break;
 
         case PAD_PROP::NONE:
+        case PAD_PROP::MECHANICAL:
             break;
         }