[MAC] DPL unresponsive while preparing for print

DPL 4 (on Mac) gets unresponsive while a (series of) print(s) is prepared for output. When a series of prints is prepared, all images are prepared first, before the first image will be printed. Both of these properties feel old fashioned.

Proposals:

  1. Change resource allocation so that DPL can be used while prints are prepared
  2. Prepare and print in parallel or make it user selectable: batch or parallel processing.