codecparsers: vc1: add API to parse slice headers.

codecparsers: vc1: add API to parse slice headers.

Add gst_vc1_parse_slice_header() function to parse slice headers as
described in 7.1.2. Slice layers are optional and allowed in advanced
profile mode only. Picture header, if available (PIC_HEADER_FLAG),
is parsed but not recorded because it shall be the same as that was
previously parsed with gst_vc1_parse_frame_header().

This fixes SA00049.vc1 conformance test.

https://bugzilla.gnome.org/show_bug.cgi?id=692388

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>