avfilter/vsrc_gfxcapture: keep cbdata object alive

Depending on the threading backend the stdlib uses, creating a
mutex/condvar can be quite expensive.
So keep this object alive in the ctx, on which we synchronize via the
uninit mutex anyway.

libavfilter/vsrc_gfxcapture_winrt.cpp

@@ -149,6 +149,7 @@ struct GfxCaptureContextCpp {
     volatile int wgc_thread_init_res { INT_MAX };
     std::recursive_mutex wgc_thread_uninit_mutex;
     volatile int wgc_thread_res { 0 };
+    std::shared_ptr<void> wgc_thread_cb_data;
 
     HWND capture_hwnd { nullptr };
     HMONITOR capture_hmonitor { nullptr };
@@ -716,11 +717,17 @@ static int run_on_wgc_thread(AVFilterContext *avctx, F &&cb)
     struct CBData {
         std::mutex mutex;
         std::condition_variable cond;
-        bool done { false };
-        bool cancel { false };
-        int ret { AVERROR_BUG };
+        bool done;
+        bool cancel;
+        int ret;
     };
-    auto cbdata = std::make_shared<CBData>();
+    auto cbdata = ctx->wgc_thread_cb_data ?
+                  std::static_pointer_cast<CBData>(ctx->wgc_thread_cb_data) :
+                  std::make_shared<CBData>();
+    ctx->wgc_thread_cb_data = cbdata;
+
+    cbdata->done = cbdata->cancel = false;
+    cbdata->ret = AVERROR_BUG;
 
     boolean res = 0;
     CHECK_HR_RET(wgctx->dispatcher_queue->TryEnqueue(