Cycles: OpenCL Separate Compilation Debug Flag

OpenCL Parallel compilation only works inside Blender. When using cycles in a different setup (standaline or other software) it failed compiling kernels as they don't have the appropriate Python API and command line arguments.

This change introduces a `running_inside_blender` debug flag, that triggers out of process compilation of the kernels. Compilation still happens in subthread that enabled the preview kernels and compilation of the kernels during BVH building

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D5439

intern/cycles/blender/blender_python.cpp

@@ -177,6 +177,8 @@ static PyObject *init_func(PyObject * /*self*/, PyObject *args)
 
   BlenderSession::headless = headless;
 
+  DebugFlags().running_inside_blender = true;
+
   VLOG(2) << "Debug flags initialized to:\n" << DebugFlags();
 
   Py_RETURN_NONE;