d3d12screencapturesrc: Fix OS handle leaks/random crash in WGC mode

Multiple DispatcherQueues per thread seems to be causing OS handle leak
and random crashes were observed. Instead of creating
thread/DispatcherQueue per GstD3D12GraphicsCapture object,
reuse only single thread and DispatcherQueue

Fixes: https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/4351
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/9153>

subprojects/gst-plugins-bad/sys/d3d12/gstd3d12graphicscapture.h

@@ -30,6 +30,8 @@ G_DECLARE_FINAL_TYPE (GstD3D12GraphicsCapture, gst_d3d12_graphics_capture,
 
 gboolean gst_d3d12_graphics_capture_load_library (void);
 
+void     gst_d3d12_graphics_capture_deinit (void);
+
 GstD3D12ScreenCapture * gst_d3d12_graphics_capture_new (GstD3D12Device * device,
                                                         HWND window_handle,
                                                         HMONITOR monitor_handle);

subprojects/gst-plugins-bad/sys/d3d12/plugin.cpp

@@ -67,6 +67,10 @@
 #include "gstd3d12upload.h"
 #endif
 
+#ifdef HAVE_WGC
+#include "gstd3d12graphicscapture.h"
+#endif
+
 /* *INDENT-OFF* */
 using namespace Microsoft::WRL;
 /* *INDENT-ON* */
@@ -74,6 +78,9 @@ using namespace Microsoft::WRL;
 static void
 plugin_deinit (gpointer data)
 {
+#ifdef HAVE_WGC
+  gst_d3d12_graphics_capture_deinit ();
+#endif
   gst_d3d12_ipc_client_deinit ();
   gst_d3d12_flush_all_devices ();
 }