Fix border painting on self-painting layers.

sky/examples/flights-app/app-toast.sky

 <import src="../../framework/sky-element/sky-element.sky" as="SkyElement" />
 
 <sky-element name="app-toast">
 <template>
   <style>
     :host {
       position: absolute;
       bottom: 32px;
       right: 32px;
       max-width: 55%;
       box-shadow: 0px 0px 12px 2px rgba(22, 22, 22, 0.4);
       background-image: linear-gradient(#E5D658, #DFCF43);
       border: 1px solid #AEA477;
       padding: 6px;
       border-radius: 2px;
       display: flex;
       align-items: center;
       font-size: 0.8em;
+      overflow: hidden;
     }
 
     .header {
       flex-shrink: 0;
       margin-right: 6px;
     }
   </style>
   <div class="header">
     <content select=".toast-icon" />
   </div>
   <div class="content">
     <content select=".toast-content" />
   </div>
 </template>
 <script>
 module.exports = class extends SkyElement {
 }.register();
 </script>
 </sky-element>