nir/opt_preamble: Treat *size as an input

nir/opt_preamble: Treat *size as an input

Some backends may wish to reserve early uniforms for internal system values, and
use the remaining space for preamble storage. In this case, it's convenient to
teach nir_opt_preamble about a reserved offset. It's logical to treat the output
*size instead of an in/out variable that nir_opt_preamble adds to. This requires
a slight change to the consumers to zero the input.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Acked-by-(with-sparkles): Asahi Lina <lina@asahilina.net>

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20562>