Cycles: Render volume by ray marching through octrees

One octree per volume per shader based on the density. In preparation
for the null scattering

intern/cycles/device/oneapi/device_impl.cpp

@@ -1292,6 +1292,7 @@ void OneapiDevice::get_adjusted_global_and_local_sizes(SyclQueue *queue,
     case DEVICE_KERNEL_SHADER_EVAL_DISPLACE:
     case DEVICE_KERNEL_SHADER_EVAL_BACKGROUND:
     case DEVICE_KERNEL_SHADER_EVAL_CURVE_SHADOW_TRANSPARENCY:
+    case DEVICE_KERNEL_SHADER_EVAL_VOLUME_DENSITY:
       preferred_work_group_size = preferred_work_group_size_shader_evaluation;
       break;