replace MapZoomWheel with MapScale

which doesn't require but subsequently ignore x and mode fields

frontend/src/Main.elm

@@ -179,7 +179,7 @@ fetchTrack start duration = Http.get
 type Msg
   = MapZoomIn
   | MapZoomOut
-  | MapZoomWheel Float Float Int
+  | MapScale Float
   | Scroll Int Int
   | PointerDown (Int, Int)
   | PointerMove (Int, Int)
@@ -201,10 +201,8 @@ newModel msg model =
         MapZoomOut ->
             { model | zoom = incZoom model.zoom -zoomStep }
 
-        MapZoomWheel x y delta ->
+        MapScale y ->
-            let dir = if y > 0
+            let dir = floor(abs(y)/y)
-                      then -1
-                      else 1
             in { model | zoom = incZoom model.zoom dir }
         Scroll x y ->
             { model | centre = translatePixels model.centre (toZoom model.zoom) (x,y) }
@@ -410,12 +408,9 @@ withSwallowing m =
     , preventDefault = True
     }
 
+-- FIXME should do something useful with deltaMode as well as deltaY
 mapWheelDecoder =
-    D.map withSwallowing
+    D.map (withSwallowing << MapScale) (D.field "deltaY" D.float)
-        (D.map3 MapZoomWheel
-             (D.field "deltaX" D.float)
-             (D.field "deltaY" D.float)
-             (D.field "deltaMode" D.int))
 
 
 viewDiv : Model -> Html Msg