Add gst_vaapi_video_format_from_string() helper function to convert from
a video format string representation to a suitable GstVideoFormat. This
is just an alias to gst_video_format_from_string() for GStreamer 1.0.x
builds, and a proper iteration over all GstVideoFormat string representations
otherwise for earlier GStreamer 0.10.x builds.
Fix new internal video format API, based on GstVideoFormat, to not
clobber with system symbols. So replace the gst_video_format_* prefix
with gst_vaapi_video_format_ prefix, even if the format type remains
GstVideoFormat.
Add gst_video_format_get_chroma_type() helper function to determine
the GstVaapiChromaType from a standard GStreamer video format. It is
possible to reconstruct that from GstVideoFormatInfo but it is much
simpler (and faster?) to use the local GstVideoFormatMap table.
