TGA now converts alpha to associated on input if requested, and on output writes
the attribute that indicates if the alpha is premultiplied or not.
Fix two issues with unassociated alpha conversion:
* PNG, TIFF and TGA input still had oiio:UnassociatedAlpha set to 1 after they
  convert the alpha to associated.
* PNG output did not take oiio:UnassociatedAlpha into account to determine if
  conversion should be done, while TIFF did.