Fix GIF export issue when user select alpha cut-off of 0
Fix premuliplied alpha rendering in Alpha Cutoff dialog

(Relates to #96)
Remove unneeded manual rendering code in batch wizard image previewer
Remove manual 32bpp compositing of original image in fxPreview control
Add support for premultiplied compositing in pdLayer class

It's much, much faster, though a number of functions need to be
rewritten to take advantage of this ability.
Remove all references to old global alpha compositing variables

(Relates to #96)

Always nice to remove globals!