review commandline optimizations for renditions

Mika did some optimizations on some transformation work for apple that reduced the process time quite a lot for very large files (was taking 50 minutes, cut it down to 10 minutes).

from Mika:
I wrote two custom image and video converters for the apple performance tests (https://github.com/nuxeo-sandbox/nuxeo-dam-optimized-converter). For images, all the different sizes are generated with only one imagemagick command such that the performance increases by 3x to 5x for the default rendentions. For videos stored in S3, instead of copying the file locally (which may require a lot of tmp space for very large video files), ffmpeg streams the file over http using a direct download URL.