Add shadow filter for selected tiles, making all-white costumes visible.

src/components/sprite-selector-item/sprite-selector-item.css

@@ -32,6 +32,10 @@
     background: $ui-white;
 }
 
+.sprite-selector-item:hover .sprite-image, .is-selected .sprite-image {
+    filter: drop-shadow(0px 0px 2px  $ui-black-transparent);
+}
+
 .sprite-image {
     margin: auto;
     user-select: none;