Cycles: Add sample-based runtime profiler that measures time spent in various parts of the CPU kernel

This commit adds a sample-based profiler that runs during CPU rendering and collects statistics on time spent in different parts of the kernel (ray intersection, shader evaluation etc.) as well as time spent per material and object.

The results are currently not exposed in the user interface or per Python yet, to see the stats on the console pass the "--cycles-print-stats" argument to Cycles (e.g. "./blender -- --cycles-print-stats").

Unfortunately, there is no clear way to extend this functionality to CUDA or OpenCL, so it is CPU-only for now.

Reviewers: brecht, sergey, swerner

Reviewed By: brecht, swerner

Differential Revision: https://developer.blender.org/D3892

intern/cycles/kernel/kernel_path_subsurface.h

@@ -32,6 +32,8 @@ bool kernel_path_subsurface_scatter(
         ccl_addr_space float3 *throughput,
         ccl_addr_space SubsurfaceIndirectRays *ss_indirect)
 {
+	PROFILING_INIT(kg, PROFILING_SUBSURFACE);
+
 	float bssrdf_u, bssrdf_v;
 	path_state_rng_2D(kg, state, PRNG_BSDF_U, &bssrdf_u, &bssrdf_v);