mpegtspacketizer: Speed up descriptor parsing/packing

mpegtspacketizer: Speed up descriptor parsing/packing

descriptors are stored as a GValueArray of GString. The downside is
that there is no way to "pass" ownership of a GValue to a GValueArray
which previously resulted in expensive copy/free of the (already expensive)
GString.

Here we estimate first the size of the GValueArray, then create it,
then directly use the GValue of that array.

Speeds up total SI parsing by ~30%