Cycles: Replace use_qbvh boolean flag with an enum-based property

This was we can introduce other types of BVH, for example, wider ones, without
causing too much mess around boolean flags.

Thoughs:

- Ideally device info should probably return bitflag of what BVH types it
  supports.

  It is possible to implement based on simple logic in device/ and mesh.cpp,
  rest of the changes will stay the same.

- Not happy with workarounds in util_debug and duplicated enum in kernel.
  Maybe enbum should be stores in kernel, but then it's kind of weird to include
  kernel types from utils. Soudns some cyclkic dependency.

Reviewers: brecht, maxim_d33

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D3011

intern/cycles/blender/blender_python.cpp

@@ -69,7 +69,7 @@ bool debug_flags_sync_from_scene(BL::Scene b_scene)
 	flags.cpu.sse41 = get_boolean(cscene, "debug_use_cpu_sse41");
 	flags.cpu.sse3 = get_boolean(cscene, "debug_use_cpu_sse3");
 	flags.cpu.sse2 = get_boolean(cscene, "debug_use_cpu_sse2");
-	flags.cpu.qbvh = get_boolean(cscene, "debug_use_qbvh");
+	flags.cpu.bvh_layout = (BVHLayout)get_enum(cscene, "debug_bvh_layout");
 	flags.cpu.split_kernel = get_boolean(cscene, "debug_use_cpu_split_kernel");
 	/* Synchronize CUDA flags. */
 	flags.cuda.adaptive_compile = get_boolean(cscene, "debug_use_cuda_adaptive_compile");