A pointer to a current SPS is already stored within Picture.
No need to add a new member `m_numSubPic`

Fixes issues with certain GDR conformance bitstreams

Having a single element reduces the number of data structures that
need to be maintained.

Should also fix memory allocation issues with PicHeader.

Ticket #1563

Ticket #1554

Not final patch. TODO:
1) Do the same in few other similar places
2) Deal with "--upscaledOutput" case which uses even more fields from
potentially deleted SPS and PPS