st/nine: Cleaning code for vs temporaries

st/nine: Cleaning code for vs temporaries

This has been a real mess up to now: the temporaries
were allocated once, and shared after that between
the different parts of the code.

To help maintaining the code, the temporaries are now
allocated and released on need.

As surprising as it could be, this patch, which was
supposed to introduce no behaviour change, actually
solved a visual bug observed on a sample program.
This was due to ureg_normalize3 polluting a temporary
variable.

Signed-off-by: Axel Davy <axel.davy@ens.fr>