feat(nvenc): implement async encode and hang recovery (#3629)

* feat(nvenc): implement async encode

* fix(video): allow NVENC to complete teardown asynchronously

src/nvenc/nvenc_d3d11.cpp

@@ -10,11 +10,19 @@
 
 namespace nvenc {
 
+  nvenc_d3d11::nvenc_d3d11(NV_ENC_DEVICE_TYPE device_type):
+      nvenc_base(device_type) {
+    async_event_handle = CreateEvent(NULL, FALSE, FALSE, NULL);
+  }
+
   nvenc_d3d11::~nvenc_d3d11() {
     if (dll) {
       FreeLibrary(dll);
       dll = NULL;
     }
+    if (async_event_handle) {
+      CloseHandle(async_event_handle);
+    }
   }
 
   bool nvenc_d3d11::init_library() {
@@ -53,5 +61,9 @@ namespace nvenc {
     return false;
   }
 
+  bool nvenc_d3d11::wait_for_async_event(uint32_t timeout_ms) {
+    return WaitForSingleObject(async_event_handle, timeout_ms) == WAIT_OBJECT_0;
+  }
+
 }  // namespace nvenc
 #endif