New plugin with an element for H.264 decoding with VA-API. This novel
approach, different from gstreamer-vaapi, uses gstcodecs library for
state handling.

The code is expected to looks cleaner because it uses VA-API without
further layers or wrappers.

* It uses the first supported DRM device as default VA display (other
  displays will be supported through user's GstContext)
* Requires libva >= 1.6
* No multiview/stereo profiles neither interlaced streams because
  gstcodecs doesn't handle them yet
* It is incompatible with gstreamer-vaapi
* Even if memory:VAMemory is exposed, it is not handled yet by any
  other element
* Caps templates are generated dynamically querying VAAPI, but YV12
  and I420 are added for system memory caps because they seem to be
  supported for all the drivers when downloading frames onto main
  memory, as they are used by xvimagesink and others, avoiding color
  conversion.
* Surfaces aren't bounded to context, so they can grow beyon...