Fix offsets in interface handling (take 2)
Fix offsets in interface handling (take 2)

There is an offset in GI function arguments between GICallbackInfo
(which includes the instance pointer as the first argument) and
GIFunctionInfo (which doesn't when the function is a method).

And of course, we got it wrong the first time.

GNOME/libpeas,gregier/libpeas,gregier/libpeas,Distrotech/libpeas,Distrotech/libpeas,GNOME/libpeas,gregier/libpeas,chergert/libpeas,Distrotech/libpeas,gregier/libpeas,chergert/libpeas,chergert/libpeas