Don't render Transition contents if they're not visible

Fixes #14

src/lib/components/transitions/TransitionRoot.svelte

@@ -168,6 +168,8 @@
   setContext(TRANSITION_CONTEXT_NAME, transitionBag);
 </script>
 
+{#if state === TreeStates.Visible}
   <TransitionChild {...$$restProps} {unmount}>
     <slot />
   </TransitionChild>
+{/if}