decoder: h264: skip SPS extension and auxiliary slice NALs.

decoder: h264: skip SPS extension and auxiliary slice NALs.

When NAL units of type 13 (SPS extension) or type 19 (auxiliary slice)
are present in a video, decoders shall perform the (optional) decoding
process specified for these NAL units or shall ignore them (7.4.1).

Implement option 2 (skip) for now, as alpha composition is not
supported yet during the decoding process.

This fixes decoding of the primary coded video in alphaconformanceG.

https://bugzilla.gnome.org/show_bug.cgi?id=703928
https://bugzilla.gnome.org/show_bug.cgi?id=728869
https://bugzilla.gnome.org/show_bug.cgi?id=724518

[skip NAL units earlier, i.e. at parsing time]
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>