#3 Alpha rounding errors in round-trip

[Brion Vibber]

Since Gimp uses non-premultiplied alpha, but the outside world
seems to want premultiplied, we have to multiply/divide and copy-
paste time.

This introduces rounding errors, which can accumulate on multiple
copy/paste cycles.

If we can generate a TIFF with non-premultiplied alpha when
copying, and paste the same cleanly, it might be possible to avoid
this at least sometimes.