Attempt to fix #35041 and #34725: cycles crash with OSL and both a 3D viewport

and preview running at the same time.

It seems there's something in OSL/LLVM that's not thread safe, but I couldn't
figure out what exactly. Now all renders share the same OSL ShadingSystem which
should avoid the problem.

intern/cycles/kernel/osl/osl_shader.cpp

@@ -127,6 +127,9 @@ static void shaderdata_to_shaderglobals(KernelGlobals *kg, ShaderData *sd,
 
 	/* clear trace data */
 	tdata->tracedata.init = false;
+
+	/* used by renderservices */
+	sd->osl_globals = kg;
 }
 
 /* Surface */