[designspace] Sometimes a wrong layer gets written

[justvanrossum]

I have a reproducer, but I need to anonimize it.

Weight axis from 1 to 1000, default at 1, with sources at 1, 30, 558 and 1000.

In my problem case, the sources for 1 and 30 contain the same as 558, instead of the correct sources. Somehow the DS backend writes the wrong layers.

[justvanrossum]

The bug is that inactive sources are stored and may overwrite the active source at the same location. So this is really a manifestation of #772.