#2662 Can't generate binary selection formats

[Donal K. Fellows]

Now that 2821962 is fixed, we're stuck with the problem that Tk (on Unix) can't do the reverse operation: deliver binary data as a selection type in 8-bit binary format; the only non-text type that we can deliver is 32-bit binary (or Atoms, but I'll ignore those). This means that if someone selects an image in Tk, it is impossible to copy it to another non-Tk application.