Batch wizard: fix potential drag/drop error
Save Image engine: fix issue with binary alpha channels and non-PNG, non-GIF formats
Lasso selections: initial build

UI stuff is not finalized, and the "bordered" type does not work, but
everything else related to the tool should be functional.  Not bad for a
few hours' work!

I had originally planned to integrate lasso and polygon selections into
the same tool, but there are some tough geometry issues involved as
polygon points should be editable, but lasso ones can't (easily) be.  So
to allow moving of some points but not others is not straightforward,
and I don't want the hassle right now... so I'm going to split them into
two separate tools, same as Photoshop.

In the meantime, lasso-ing works very well.  All Select menu items work,
Undo/Redo works, and lasso-type selections can be exported and imported
on any arbitrary image.  Antialiasing and feathering can be
non-destructively modified, and internal/external selection type can
also be non-destructively swapped.  I haven't decided if the "border"
type is worth the trouble yet, but I'm leaning toward simply disabling
it for lasso selections.  It can always be triggered manually from the
Select menu, using raster-detection methods (instead of vector ones).

GDI+ does all the heavy lifting as far as drawing the selection mask,
and PD's existing routines handle all integration with tools and
dialogs.  All in all, I'm quite happy with how straightforward it was to
add this!