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>
 
-<TransitionChild {...$$restProps} {unmount}>
+{#if state === TreeStates.Visible}
-  <slot />
+  <TransitionChild {...$$restProps} {unmount}>
-</TransitionChild>
+    <slot />
+  </TransitionChild>
+{/if}