TJBench: Improve consistency of results Given that libjpeg-turbo can often process hundreds of megapixels/second on modern hardware, the default of one warmup iteration was essentially meaningless. Furthermore, the -warmup option was a bit clunky, since it required some foreknowledge of how fast the benchmarks were going to execute. This commit introduces a 1-second warmup interval for each benchmark by default, and the -warmup option has been retasked to control the length of that interval.