fix #157: problem with live multi-camera selection

When you select the first camera in the drop-down on a grid square
that isn't the top left, then select a camera in the top-left, it
behaves strangely.

The root cause is that I had a dumb mistake in how I assigned React
keys. I used the camera index when a camera is selected, and the
inverse of the selected index when one is. But 0 == -0!

ui/src/Live/Multiview.tsx

@@ -164,8 +164,8 @@ const Multiview = (props: MultiviewProps) => {
     // WebSocket connections and buffers.
     //
     // When no camera is selected, use the index within selected. (Actually,
-    // its negation, to disambiguate between the two cases.)
-    const key = e ?? -i;
+    // -1 minus the index, to disambiguate between the two cases.)
+    const key = e ?? -1 - i;
     return (
       <Monoview
         key={key}