range: use border box for slider area

Since we can use negative margins, we should not use the margin box
for the slider area. Use the border box instead, since that's what is
typically mapped to the visible area.

gtk/gtkrange.c

@@ -1279,7 +1279,7 @@ gtk_range_get_slider_range (GtkRange *range,
 
   priv = range->priv;
 
-  gtk_css_gadget_get_margin_box (priv->slider_gadget, &slider_alloc);
+  gtk_css_gadget_get_border_box (priv->slider_gadget, &slider_alloc);
 
   if (priv->orientation == GTK_ORIENTATION_VERTICAL)
     {
@@ -2469,7 +2469,7 @@ coord_to_value (GtkRange *range,
   gint    slider_length;
   GtkAllocation slider_alloc, trough_alloc;
 
-  gtk_css_gadget_get_margin_box (priv->slider_gadget, &slider_alloc);
+  gtk_css_gadget_get_border_box (priv->slider_gadget, &slider_alloc);
   gtk_css_gadget_get_content_box (priv->trough_gadget, &trough_alloc);
 
   if (priv->orientation == GTK_ORIENTATION_VERTICAL)
@@ -2512,7 +2512,7 @@ gtk_range_key_press (GtkWidget   *widget,
   device = gdk_event_get_device ((GdkEvent *) event);
   device = gdk_device_get_associated_device (device);
 
-  gtk_css_gadget_get_margin_box (priv->slider_gadget, &slider_alloc);
+  gtk_css_gadget_get_border_box (priv->slider_gadget, &slider_alloc);
 
   if (gtk_gesture_is_active (priv->drag_gesture) &&
       device == gtk_gesture_get_device (priv->drag_gesture) &&
@@ -2549,7 +2549,7 @@ gtk_range_long_press_gesture_pressed (GtkGestureLongPress *gesture,
   GtkAllocation slider_alloc;
 
   gtk_range_update_mouse_location (range);
-  gtk_css_gadget_get_margin_box (priv->slider_gadget, &slider_alloc);
+  gtk_css_gadget_get_border_box (priv->slider_gadget, &slider_alloc);
 
   if (priv->mouse_location == MOUSE_SLIDER && !priv->zoom)
     {
@@ -2603,7 +2603,7 @@ gtk_range_multipress_gesture_pressed (GtkGestureMultiPress *gesture,
   priv->mouse_y = y;
 
   gtk_range_update_mouse_location (range);
-  gtk_css_gadget_get_margin_box (priv->slider_gadget, &slider_alloc);
+  gtk_css_gadget_get_border_box (priv->slider_gadget, &slider_alloc);
 
   g_object_get (gtk_widget_get_settings (widget),
                 "gtk-primary-button-warps-slider", &primary_warps,
@@ -2780,7 +2780,7 @@ update_slider_position (GtkRange *range,
   gint i;
   GtkAllocation slider_alloc, trough_alloc;
 
-  gtk_css_gadget_get_margin_box (priv->slider_gadget, &slider_alloc);
+  gtk_css_gadget_get_border_box (priv->slider_gadget, &slider_alloc);
   gtk_css_gadget_get_margin_box (priv->trough_gadget, &trough_alloc);
 
   if (priv->zoom)