"Build all thumbnails/resizes" should parallelize for multicpu, multicore systems

Building all thumbnails / resizes is a very long and CPU intensive process which only uses a single thread. It would be great if this could be forked or otherwise parallelized to utilize the many cores available on most servers. I've tried opening multiple tabs to the same page but that results in errors.

I'm running php as an Apache module with Apache in prefork MPM mode so I'm not sure if the worker (threaded) model or PHP as a CGI differs but I'm pretty sure even those configurations would still call `convert` (Imagemagick) in series, not parallel.