MANAGER: Add a method for each descriptor output size computation

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
MANAGER: Add a performance test

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
MANAGER: Now all computation are copied through an intermediary buffer

This will allow reusing various common results between descriptors.
Part of #29.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
DESCRIPTORS: Clean descriptors interface

Minor style fixes.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>