Gstreamer modifier playback notice

On DG2 platform, gstreamer-va/msdk modifier playback needs pay attention to

1. Need to start X-server

   /usr/bin/Xorg :0

2. check OpenGL mesa version

   glxinfo | grep OpenGL

3. set display port

   export DISPLAY=:0.0

4. need add environment variable

   GST_GL_PLATFORM=egl GST_GL_API=gles2

5. Perhaps you need to manually build the latest mesa

   meson -Dlibdir=lib/x86_64-linux-gnu -Dplatforms=x11 -Dgallium-drivers=swrast,iris -Dvulkan-drivers= -Dllvm=disabled -Dgbm=enabled -Dglx=auto -Dprefix=/usr -Dglvnd=true build