[Fixes bug #707587] Make a copy of the target layer before we start, and change it directly so we correctly change alpha values.  Then copy it using "Source" so alpha values are replaced in the original (but only in the selection).
[Fixes bug #707587] Make a copy of the target layer before we start, and change it directly so we correctly change alpha values.  Then copy it using "Source" so alpha values are replaced in the original (but only in the selection).

PintaProject/Pinta,Mailaender/Pinta,PintaProject/Pinta,Fenex/Pinta,Fenex/Pinta,jakeclawson/Pinta,Mailaender/Pinta,jakeclawson/Pinta,PintaProject/Pinta