Bundle convert - and montage-calls

[tokee]

Observations of juxta suggests that as much as 1/3 of all processing power used is by the kernel. This is probably due to the large amount of calls to convert and montage. If these calls could be bundled, the system-overhead of calling outside of bash as well as any startup-overhead for the image manipulation programs themselves could be minimized.

ImageMagic seems to support something like that, but it needs to be investigated further: http://www.imagemagick.org/Usage/basics/#parenthesis