Multithread-write fix for OurPaint

Recently I have been using OurPaint to do extra images for my comic, and I have encountered an issue where a empty layer could cause reading to fail, turns out to be the segmented write implementation skipped writing a size number in the file.

Now everything is fixed, and multithread write should be default ON starting in next version. (As long as I don’t find other instabilities)

Anyway, stay tuned for the next version :D