bug#70455: [PATCH] Add new user option `preview-leave-open-previews-visible'

[Paul Nelson]

With the new user option disabled (default), behavior is exactly as before:

- When you enter a preview, it is temporarily replaced by a
placeholder icon together with the tex code.
- If you exit the preview without editing, the preview reappears and
the tex code disappears.
- When you edit the tex code, the preview is gone forever.
- When you regenerate a preview, a construction sign briefly appears.

With the new user option disabled, behavior is instead:

- When you enter a preview, it remains visible.  The tex code appears
below the preview.
- When you edit the tex code, the preview does not disappear.  The tex
code persists if you then exit.
- When you regenerate a preview, there is no construction sign, but
instead a direct transition from the old image to the new one.

The advantage is that while editing a formula, you (and anyone else
watching your screen) can continue to look at its rendered form.  If
the user option "preview-protect-point" is active, then a quick "C-c
C-p C-p" or "C-c C-p C-e" will update the rendered form, without
flickering.

I would welcome suggestions for better names than
`preview-leave-open-previews-visible'.

Thanks, best,

Paul

0001-Allow-opened-previews-to-remain-visible.patch
Description: Source code patch