rendering slices for different file types

It would be nice if slice rendering and assembly could produce files other than .iff. Our comps are never set up for iffs, and we want to render hi-rez stills at the end of a project without redoing the comps.

The documentation seems to say that slicing and reassembly works with iff, rla, tga, and bmp, but the template is hardwired for iff.

While the 8-bit formats are easily interconverted, we want to use EXR.