Fix Erase composite op to handle the selections properly

Using blend() for applying a mask is surely a bad idea. Don't know why it
was used, but it was dated to 2007. Thanks Timothee for pointing out this
bug.

<Animtim> dmitryK: hey yesterday I noticed a nasty bug, and just took time to understand what trigger it : if I have a "transparent" selection, and try to paint in erase mode inside, it makes bad tip tiles artifacts
<Animtim> (to reproduce: create paint something at low opacity, do select opaque, and try to erase..)
<Animtim> erasing with any kind of brush engine makes the bug

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