Minor correction on syntax coloring

The semantics were changed a bit in https://github.com/fsprojects/VisualFSharpPowerTools/commit/30a73458fd890e20ca17f84097abf6c38a957eaa. The correction is that we construct the span on the old snapshot and translate it to a span on the new snapshot.
I tried such translation of all spans several months ago. It's *very* expensive. Have you profiled it?