decoder: h264: fix activation of picture and sequence parameters.

decoder: h264: fix activation of picture and sequence parameters.

At the time the first VCL NAL unit of a primary coded picture is found,
and if that NAL unit was parsed to be an SPS or PPS, then the entries
in the parser may have been overriden. This means that, when the picture
is to be decoded, slice_hdr->pps could point to an invalid (the next)
PPS entry.

So, one way to solve this problem is to not use the parser PPS and
SPS info but rather maintain our own activation chain in the decoder.

https://bugzilla.gnome.org/show_bug.cgi?id=724519
https://bugzilla.gnome.org/show_bug.cgi?id=724518