Fix some alignment issues

LRN requested to merge lrn/alignfixes into master
  • 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

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