Cycles: Add fundamentals to test constant folding

This commit adds some easy to use way to check whether the graph
was properly optimized. The idea is based on using mock glog sync
which keeps track on all messages and expects specific log message
to appear in the logging.

This means each optimization step should report that it optimized
something (specifying what exactly was optimized) and what was used
for optimization.

There's also some ease-ish way to define shader networks which we
might want to make a bit more global and available for all tests
in the future.

For now only RGBToBW node is covered as an example, really hope
to get help from active guys in the community to finish covering
all existing optimization cases. Ready to fix any possible issues
with builder when needed tho :)

intern/cycles/render/constant_fold.cpp

@@ -18,6 +18,7 @@
 #include "graph.h"
 
 #include "util_foreach.h"
+#include "util_logging.h"
 
 CCL_NAMESPACE_BEGIN
 
@@ -39,6 +40,7 @@ bool ConstantFolder::all_inputs_constant() const
 
 void ConstantFolder::make_constant(float value) const
 {
+	VLOG(1) << "Replacing " << node->name << " with constant " << value << ".";
 	foreach(ShaderInput *sock, output->links) {
 		sock->set(value);
 	}