Background of oval selection doesn't get replaced with transparency

[Lehonti]

Fixes #1385

With a caveat: when moving parts of a translucent image, the behavior is different.

Before (replacement):

After (overlap):

And for reference, issue #840 changed the behavior to replace the area below with the selection, so this would be some kind of rollback, but if this is not desired we can try to find some way to fix the bug without changing the behavior.

[cameronwhite]

Yeah, it seems like this would essentially revert the changes for #840 / #819. I'm not against doing that if we can't come up a solution, since the regression it caused with pasting is annoying, but I'd like to see if we can find a solution for #1385 and the remaining todos from #819

[pedropaulosuzuki]

I personally would expect 'replace' on paste, instead of 'alpha over', unless it was pasted into a new layer. The ideal solution would be to mask the selection without using the alpha channel, but a separate bitmap, so we could paste non-rectangular selections, but without messing the alpha channel. If that isn't possible, maybe spawning a new layer on paste wouldn't be the worst of the compromises, as this wouldn't destroy the underlying pixels, but could create friction for some users. We could also add a toggle that changed this behavior in the toolbar in some way. I feel like just doing alpha over could be a regression, in some sense.