Fix Stage to scrolling issue (Addresses pebble#163)

Stage was using the same content size resizing logic as Card which
requires the same fix. Don't update the content size unless necessary
since updating the content size cancels the scroll animation on certain
firmware versions.

src/simply/simply_stage.c

@@ -339,7 +339,10 @@ static void layer_update_callback(Layer *layer, GContext *ctx) {
   if (self->window.is_scrollable) {
     frame.origin = GPointZero;
     layer_set_frame(layer, frame);
-    scroll_layer_set_content_size(self->window.scroll_layer, frame.size);
+    const GSize content_size = scroll_layer_get_content_size(self->window.scroll_layer);
+    if (!gsize_equal(&frame.size, &content_size)) {
+      scroll_layer_set_content_size(self->window.scroll_layer, frame.size);
+    }
   }
 }