Shouldn't this check if the wavelength dependence is separable here?  It seems like this would be the `draw` function that would be used for `Chromatic` objects, which are separable.  For them (and other separable `ChromaticObject`s, I would have thought that integrating the SED over the bandpass first (with `int1d`) and then multiplying the gsobject by that factor would be much faster than the image integration.