Remove transition on sprite tiles to make placholder not flash black.

Also use white as the background since it gives the same impact and will not look bad if the filter isn't applied or takes a while to be applied

src/components/asset-panel/selector.css

@@ -72,6 +72,6 @@ $fade-out-distance: 100px;
 
 
 .list-item.placeholder {
-    background: black;
+    background: white;
     filter: opacity(15%) brightness(0%);
 }

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

@@ -18,7 +18,6 @@
 
     text-align: center;
     cursor: pointer;
-    transition: 0.25s ease-out;
 
     user-select: none;
     touch-action: none;