Addresses performance issue for 12-bit video

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

Set m_numSubPic to a value signaled in SPS for the layer.
Otherwise an error is triggered when an unavailable picture
happens to be in the reference list for GDR picture partitioned into
more than 1 sub-pictures.

Error message is: "TMVP cannot be enabled in pictures
that have no reference pictures with the same resolution"

Fixes issues with certain GDR conformance bitstreams

The current code misses the first character of the string for some reason. Maybe a compiler problem.
Using a local variable fixed this.

this should fix ticket #1643