decoder: h264: add support for NALU "alignment" optimization.

decoder: h264: add support for NALU "alignment" optimization.

We can avoid scanning for start codes again if the bitstream is fed
in NALU chunks. Currently, we always scan for start codes, and keep
track of remaining bits in a GstAdapter, even if, in practice, we
are likely receiving one GstBuffer per NAL unit. i.e. h264parse with
"nal" alignment.

https://bugzilla.gnome.org/show_bug.cgi?id=723284

[use gst_adapter_available_fast() to determine the top buffer size]
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>