Add Future::TryGet() method

This takes a mutex (that should be held at call time). It returns the
future's value if it's available and otherwise unlocks the mutex, tries to
do some work, and then returns with the mutex re-locked.  This allows
mutex-protected future harvesting to work without deadlock...

src/pbrt/util/parallel.h

@@ -362,6 +362,15 @@ class Future {
         Wait();
         return fut.get();
     }
+    pstd::optional<T> TryGet(std::mutex *mutex) {
+        if (IsReady())
+            return Get();
+
+        mutex->unlock();
+        DoParallelWork();
+        mutex->lock();
+        return {};
+    }
 
     void Wait() {
         while (!IsReady() && DoParallelWork())