Pass the ?pool query param through to the backend. (#2246)

Pass the ?pool query param through to the backend.

This will slice off the ?pool= param and pass it to the WebSocket
request, which requests that the Zoo API use a particular pool of
engines. This isn't something any users of the zoo api require; but it's
needed for the internal engine Zoo development workflow. This may be
used in the future, but for now this'll be always enabled. Passing any
value in the production servers will result in a "no backend" error for
now.

src/hooks/useSetupEngineManager.ts

@@ -9,10 +9,12 @@ export function useSetupEngineManager(
   streamRef: React.RefObject<HTMLDivElement>,
   token?: string,
   settings = {
+    pool: null,
     theme: Themes.System,
     highlightEdges: true,
     enableSSAO: true,
   } as {
+    pool: string | null
     theme: Themes
     highlightEdges: boolean
     enableSSAO: boolean
@@ -35,6 +37,12 @@ export function useSetupEngineManager(
 
   const hasSetNonZeroDimensions = useRef<boolean>(false)
 
+  if (settings.pool) {
+    // override the pool param (?pool=) to request a specific engine instance
+    // from a particular pool.
+    engineCommandManager.pool = settings.pool
+  }
+
   useLayoutEffect(() => {
     // Load the engine command manager once with the initial width and height,
     // then we do not want to reload it.