Add single-threaded config to Filament. (#130)

* Add single-threaded config to Filament.

This adds a tick method to Engine and disables a couple components
in Renderer (FrameSkipper and FrameInfoManager).

This will make it easier to support WebGL, and will allow us to remove
some of the command buffer debugging stuff that we added for Vulkan.

* tick => execute, and other review feedback

* Restore the ASSERT for FFence::wait.

libs/utils/src/JobSystem.cpp

@@ -120,7 +120,7 @@ JobSystem::JobSystem(size_t threadCount, size_t adoptableThreadsCount) noexcept
             threadCount = hwThreads - 1;
         }
     }
-    threadCount = std::min(size_t(32), threadCount);
+    threadCount = std::min(size_t(UTILS_HAS_THREADING ? 32 : 0), threadCount);
 
     mThreadStates = aligned_vector<ThreadState>(threadCount + adoptableThreadsCount);
     mThreadCount = uint16_t(threadCount);