Windows: setup a default handler for ImeSetInputScreenPosFn so IME (Japanese/Chinese) inputs are positioned correctly.

User need to set io.ImeWindowHandle

examples/directx9_example/main.cpp

@@ -240,6 +240,7 @@ void InitImGui()
     io.KeyMap[ImGuiKey_Z] = 'Z';
 
     io.RenderDrawListsFn = ImImpl_RenderDrawLists;
+    io.ImeWindowHandle = hWnd;
     
     // Create the vertex buffer
     if (g_pd3dDevice->CreateVertexBuffer(10000 * sizeof(CUSTOMVERTEX), D3DUSAGE_DYNAMIC | D3DUSAGE_WRITEONLY, D3DFVF_CUSTOMVERTEX, D3DPOOL_DEFAULT, &g_pVB, NULL) < 0)