This patch makes the following changes, having no effect on behavior:

- The variable preview--region-begin, introduced in the fix to
Bug#62445, is replaced with preview-current-region, which is a cons
cell (begin . end) keeping track of the complete region.
- That variable is set to nil in preview-parse-messages.

The effect is to give a way for (e.g.) external packages to keep track
of when and where previews are being generated.

Thanks, best,

Paul