Fix some alignment issues

* Use GRAPHENE_ALIGN16 to ensure that GtkSnapshotState size
  is a multiple of 16
* Use G_STATIC_ASSERT instead of g_return_val_if_fail to make
  the wrong size (if it turns out to be wrong) be a compile-time
  erorr instead of a runtime one

gtk/gtksnapshot.c

@@ -67,8 +67,6 @@ gtk_snapshot_state_array_new (void)
 {
   GtkRealSnapshotStateArray *array;
 
-  g_return_val_if_fail (sizeof (GtkSnapshotState) % 16 == 0, NULL);
-
   array = g_slice_new (GtkRealSnapshotStateArray);
 
   array->data      = NULL;

gtk/gtksnapshotprivate.h

@@ -40,6 +40,7 @@ typedef GskRenderNode * (* GtkSnapshotCollectFunc) (GtkSnapshot      *snapshot,
                                                     GskRenderNode   **nodes,
                                                     guint             n_nodes);
 
+GRAPHENE_ALIGN16
 struct _GtkSnapshotState {
   guint                  start_node_index;
   guint                  n_nodes;
@@ -91,6 +92,8 @@ struct _GtkSnapshotState {
   } data;
 };
 
+G_STATIC_ASSERT (sizeof (GtkSnapshotState) % 16 == 0);
+
 /* This is a nasty little hack. We typedef GtkSnapshot to the fake object GdkSnapshot
  * so that we don't need to typecast between them.
  * After all, the GdkSnapshot only exist so poor language bindings don't trip. Hardcore