Build unnecessarily compares tiny (_tn) images

[martinbudden]

The build process compares the _tn images and then deletes the difference files. Although the _tn files are small and so quicker to compare, it still takes time and slows down the build.

[nophead]

I will have a look at the times when I am at home but magick takes no time on my system iirc.

[nophead]

The magic compares typically take around 100ms, so I don't see any reason to change this. OpenSCAD takes nearly 2 seconds to do nothing and often takes minutes, so it totally dwarfs the times.

In a build that takes 423 seconds to do the assembly images it only took 2 seconds to do all the small image compares and 6.7 to do the large ones.

[nophead]

I just pushed a change that logs all the command times in cmd_times.txt.

Please can you confirm the times to run magick are negligible on your system as well.