Constant item objects need to support several val_str() calls,
which means we cannot use string buffers owned by bg_resbuf_mgr
(it will free results from previous calls on re-invokation of val_str()).

Do geometry computations using temporary String objects, with buffers
owned by the geometry subsystem, and then copy the result to the
output result argument in the val_str() functions.

Change-Id: Ibaceda9634463abb2c9d718c93f8ab4449f1cd95
(cherry picked from commit d07e96320d0be0d5522b89483460c6d7adef85e2)