The built-in video parsers elements are built into a single DSO named
libgstvaapi_parse.so. The various video parsers could be accessed as
vaapiparse_CODEC.
For now, this only includes a modified version of h264parse so that to
support H.264 MVC encoded streams.
Add libvpx submodule that tracks the upstream version 1.3.0. This is
needed to build a libgstcodecparsers_vpx.so library with all symbols
placed into the GSTREAMER namespace.