Add RunOnWndMsgThread to run arbitrary code on the window message thread

src/core/hle/D3D8/Direct3D9/Direct3D9.cpp

@@ -71,6 +71,7 @@
 #include <assert.h>
 #include <process.h>
 #include <clocale>
+#include <functional>
 #include <unordered_map>
 #include <thread>
 
@@ -557,6 +558,16 @@ const char *CxbxGetErrorDescription(HRESULT hResult)
 	return nullptr;
 }
 
+static constexpr UINT WM_CXBXR_RUN_ON_MESSAGE_THREAD = WM_USER+0;
+
+// A helper function to run any code on a window message thread
+// Used for those D3D9 function which *must* be ran on this particular thread
+static void RunOnWndMsgThread(const std::function<void()>& func)
+{
+	const void* param = &func;
+	SendMessage(g_hEmuWindow, WM_CXBXR_RUN_ON_MESSAGE_THREAD, reinterpret_cast<WPARAM>(param), 0);
+}
+
 
 const char *D3DErrorString(HRESULT hResult)
 {
@@ -2109,6 +2120,13 @@ static LRESULT WINAPI EmuMsgProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lPar
         }
         break;
 
+		case WM_CXBXR_RUN_ON_MESSAGE_THREAD:
+		{
+			auto func = reinterpret_cast<const std::function<void()>*>(wParam);
+			(*func)();
+			return 0;
+		}
+
         default:
             return DefWindowProc(hWnd, msg, wParam, lParam);
     }