codecparsers: h264: store SPS's and subset SPS's separately

Submitted by Orestis Floros

Link to original bug (#786797)

Description

From the H264 spec: H.7.4.1.2.1 - Order of MVC sequence parameter set RBSPs and picture parameter set RBSPs and their activation:

NOTE 5 – A decoder must be capable of simultaneously storing the contents of the sequence parameter sets and subset sequence
parameter sets for all values of seq_parameter_set_id. The content of the sequence parameter set with a particular value of
seq_parameter_set_id is overwritten when a new sequence parameter set NAL unit with the same value of seq_parameter_set_id is
received, and the content of the subset sequence parameter set with a particular value of seq_parameter_set_id is overwritten when
a new subset sequence parameter set NAL unit with the same value of seq_parameter_set_id is received.

This should also be required for parsing SVC streams.

Some discussion in bug 732266:
https://bugzilla.gnome.org/show_bug.cgi?id=732266#c36