Add a COLUMN_PRECISION constant to get non-overlapping seek ranges

Set it to 0.95 for now (choose from the middle 95% of the frame).
Shouldn't hurt much performance-wise, and improves quality.
Hack mpv-nordlicht to go fullscreen and show the barcode on startup
Don't cheat when compressing

It's not much on an performance benefit, but much uglier.
Downsample with an - also quite primitive - averaging filter. Fixes #18.