h264parse: introduce new state tracking variables.

h264parse: introduce new state tracking variables.

Improve parser state tracking by introducing new flags reflecting
it: "got-sps", "got-pps" and "got-slice". This is an addition for
robustness purposes.

Older have_sps and have_pps variables are kept because they have
a different meaning. i.e. they are used for deciding on when to
submit updated caps or not, and rather mean "have new SPS/PPS to
be submitted?"

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