Reviewers: ,

Description:
1. Refactor "bind" to simplify the handling of globals.
2. Bug fix (exec.cpp:309) -- zero out local strings, the one place where
uninitialized values in the heap can crash the renderer.
3. Infrastructure for eventual "rebind" optimization, including tracking
the statistics on it and allowing an option to enable/disable.


Please review this at http://codereview.appspot.com/195042/show

Affected files:
src/liboslexec/context.cpp
src/liboslexec/exec.cpp
src/liboslexec/oslexec_pvt.h
src/liboslexec/shadingsys.cpp
src/testshade/testshade.cpp