fix: make backfacing shadeop indicate backfacing shader-global is needed (#1827)

The backfacing shadeop now directly accesses the backfacing field
on the shader globals rather than calculating it from N and I.
This updates the shader-globals-needed query to reflect that
change.

Signed-off-by: Stephen Friedman <sfriedma@pixar.com>

Author: sfriedmapixar

src/liboslexec/runtimeoptimize.cpp

@@ -3348,8 +3348,7 @@ RuntimeOptimizer::run()
                     m_unknown_closures_needed = true;
                 }
             } else if (op.opname() == u_backfacing) {
-                m_globals_needed.insert(u_N);
-                m_globals_needed.insert(u_I);
+                m_globals_needed.insert(u_backfacing);
             } else if (op.opname() == u_calculatenormal) {
                 m_globals_needed.insert(u_flipHandedness);
             } else if (op.opname() == u_getattribute) {

testsuite/globals-needed/ref/out.txt

@@ -1,2 +1,2 @@
-Need 2 globals:  I N
+Need 1 globals:  backfacing
 Need 2 globals:  P flipHandedness