Fix OptiX acceleration structure failing to build in viewport

The `optixAccelBuild` API throws an error when the property to get compacted size is passed in
without the `OPTIX_BUILD_FLAG_ALLOW_COMPACTION` flag set. This is not currently hit
because `background` is always true (set in `mem_alloc`), but would become an issue once that
is sorted out, so fixing it now to be safe.

intern/cycles/device/device_optix.cpp

@@ -1188,8 +1188,8 @@ class OptiXDevice : public Device {
                                            out_data,
                                            sizes.outputSizeInBytes,
                                            &out_handle,
-                                           &compacted_size_prop,
-                                           1));
+                                           background ? &compacted_size_prop : NULL,
+                                           background ? 1 : 0));
 
     // Wait for all operations to finish
     check_result_cuda_ret(cuStreamSynchronize(NULL));