TGA now converts alpha to associated on input if requested, and to unassociated
on output. TGA does support storing both types of alpha but many implementations
seem to assume unassociated so always write it that way.
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.