fix(overlay): ensure container covers entire screen

Author: jelbourn

src/lib/core/overlay/_overlay.scss

@@ -18,8 +18,8 @@
     // The container should be the size of the viewport.
     top: 0;
     left: 0;
-    height: 100%;
-    width: 100%;
+    height: 100vh;
+    width: 100vw;
     z-index: $md-z-index-overlay-container;
   }