straight alpha option on export

[ashf]

Would be nice if you could add  alpha option to select premultiplied/straight for alpha capable format such as PNG/TIF/TGA.
it would make round trip editing with other apps easier.

Straight alpha means RGB channels including transparent part will remain untouched without filled with a color.

if I open a TGA that's with straight alpha and export as TGA, it will be premultiplied which is pain.