TGA output now always writes unassociated alpha, even if the input is associated.
This is because common applications like Gimp, Photoshop and Preview will assume
TGA files to have unassociated alpha.
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.