Reviewers: ,

Description:
This change allows a debug mode (set by attribute("debugnan",1)) that
checks every value written by each instruction, and if any nan or inf
values are found, issues a warning pointing to exactly which shader
source line it happened on. It's quite expensive, nobody should use it
for ordinary rendering, but it's real helpful if you are trying to debug
NaNs in your renderer, this will quickly narrow it down to the
particular instruction.

(SPI folks: a corresponding renderer-side review will come to you
shortly.)

Please review this at http://codereview.appspot.com/194142/show

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