Control the active state of pool. When the pool is inactive, new calls to Buffer.poolAcquireBuffer will return with GST_FLOW_FLUSHING.
Activating the bufferpool will preallocate all resources in the pool based on the configuration of the pool.
Deactivating will free the resources again when there are no outstanding buffers. When there are outstanding buffers, they will be freed as soon as they are all returned to the pool.