only enable precipitation if it's enabled in the gui

(/sim/rendering/precipitation-gui-enabled) *and* if the aircraft wants it
(/sim/rendering/precipitation-aircraft-enabled). Some aircraft model rain
on the windshield and want to turn rain off for interior views.

gui/dialogs/rendering.xml

@@ -90,7 +90,7 @@
     <checkbox>
       <halign>left</halign>
       <label>Precipitation</label>
-      <property>/sim/rendering/precipitation-enable</property>
+      <property>/sim/rendering/precipitation-gui-enable</property>
       <binding>
         <command>dialog-apply</command>
       </binding>