old issue, the formulas here were never quite right, should all work ok now
with byte and float images.
Some differences:
* Colors with zero alpha from the background will never have an influence, so
you don't get alpha fringes when painting over such areas. This does give
hard edges when looking at the RGB channels alone, but there's no way to
avoid that and fringes at the same time, same behavior as other painting apps.
* Add/Subtract/Multiply/Lighten/Darken now leave the alpha channel unchanged
and work only the RGB channels, again same behavior as many other apps.
* Erase/Add alpha now compensates for premultiplied float images to keep the
straight RGB colors the same.
Next: fix projection painting.
8f9150871c