Use a more noticeable shadow when displaying one tile on top another

So that the local tile in 1:1 calls is more discernable against the background, especially when both participants have their video off

src/video-grid/VideoTileContainer.tsx

@@ -47,6 +47,7 @@ interface Props {
   opacity?: SpringValue<number>;
   scale?: SpringValue<number>;
   shadow?: SpringValue<number>;
+  shadowSpread?: SpringValue<number>;
   zIndex?: SpringValue<number>;
   x?: SpringValue<number>;
   y?: SpringValue<number>;