Hide outfit thumbnail in wardrobe after loading

Oops, it was possible after saving an outfit to get into a state where we would show the `<OutfitThumbnailIfCached />` behind the outfit even after it was saved, and then removing items would look weird until auto-saving caught up.

We had used the `backdrop` property because we wanted smoother partial load-ins, but for now I'm just fixing this by switching it to `placeholder`, which already has the right loading-only behavior.

This was also the only call site for `backdrop`, so I've removed it!

src/app/WardrobePage/WardrobePreviewAndControls.js

@@ -29,7 +29,7 @@ function WardrobePreviewAndControls({
     appearanceId: outfitState.appearanceId,
     wornItemIds: outfitState.wornItemIds,
     onChangeHasAnimations: setHasAnimations,
-    backdrop: <OutfitThumbnailIfCached outfitId={outfitState.id} />,
+    placeholder: <OutfitThumbnailIfCached outfitId={outfitState.id} />,
     "data-test-id": "wardrobe-outfit-preview",
   });
 
@@ -59,11 +59,6 @@ function WardrobePreviewAndControls({
  * outfit thumbnail instantly while everything else loads. But on direct
  * navigation, this does nothing, and we just wait for the preview to load in
  * like usual!
- *
- * We even use it as our "backdrop", so the thumbnail actually _always_ renders
- * if possible. This makes loading look even smoother, by letting individual
- * layers load in on top of the thumbnail.
- * TODO: Can this become a perf problem on WIP outfits?
  */
 function OutfitThumbnailIfCached({ outfitId }) {
   const { data } = useQuery(

src/app/components/OutfitPreview.js

@@ -49,7 +49,6 @@ export function useOutfitPreview({
   appearanceId = null,
   isLoading = false,
   placeholder = null,
-  backdrop = null,
   loadingDelayMs,
   spinnerVariant,
   onChangeHasAnimations = null,
@@ -98,7 +97,6 @@ export function useOutfitPreview({
         loading={isLoading || loading || loading2}
         visibleLayers={loadedLayers}
         placeholder={placeholder}
-        backdrop={backdrop}
         loadingDelayMs={loadingDelayMs}
         spinnerVariant={spinnerVariant}
         onChangeHasAnimations={onChangeHasAnimations}
@@ -120,7 +118,6 @@ export function OutfitLayers({
   loading,
   visibleLayers,
   placeholder = null,
-  backdrop = null,
   loadingDelayMs = 500,
   spinnerVariant = "overlay",
   doTransitions = false,
@@ -185,18 +182,6 @@ export function OutfitLayers({
           data-loading={loading ? true : undefined}
           {...props}
         >
-          {backdrop && (
-            <FullScreenCenter>
-              <Box
-                width="100%"
-                height="100%"
-                maxWidth="600px"
-                maxHeight="600px"
-              >
-                {backdrop}
-              </Box>
-            </FullScreenCenter>
-          )}
           {placeholder && (
             <FullScreenCenter>
               <Box