Mark the following functions are internal, i.e. private to the vaapi plug-in:
- gst_vaapi_video_buffer_pool_get_type()
- gst_vaapi_video_converter_glx_get_type()
- gst_vaapi_video_converter_glx_new()
Make sure libgstvaapi core decoding library doesn't include un-needed
dependencies. So, move out GstVaapiVideoConverterGLX to plugins instead.
Besides, even if the vaapisink element is not used, we are bound to have
a correctly populated GstSurfaceBuffer from vaapidecode.
Also clean-up the file along the way.