IME seems not support in inputtext

[wuqi]

Input Method Editors can't use in inputtext,but it's ok in gltf backend on windows.

I find the code in glfw backend ,need add something like this:

  #if defined(_WIN32)
      ImGui::GetMainViewport()->PlatformHandleRaw = (void*)glfwGetWin32Window(bd->Window);
  #endif