Fix resize the window correctly on dx9 backend

Maybe, we need to release some more resources when reset the device.
You need to surround this call with an ifdef, like this:

    #ifdef _WIN32  
    D3D9_Resize(0);
    #endif

Also, add a regular "if" that checks that the g_Config.iGPUBackend is set to GPU_BACKEND_DIRECT3D9, we don't want this resizing code to run in OpenGL mode.