As I've thought about this more I've realized that I'm being silly. There is no way to map a sphere to a single linear texture without some discontinuities. But a renderer that cares about getting it right can easily take the provided image, in whatever format it appears, and convert it into a cube map or whatever is convenient.
That said, it would be preferable to use a projection that allows for linear texturing with only *one* discontinuity - texture creators can then arrange to place that discontinuity somewhere that's unlikely to cause problems.
That said, it would be preferable to use a projection that allows for linear texturing with only *one* discontinuity - texture creators can then arrange to place that discontinuity somewhere that's unlikely to cause problems.