Below you will find a patch to extend show-paren-mode with a new
`prelude' style.  It's an extension of `parenthesis' style.  If the
matching opening paren is not visible, it will show the line of text
containing it as an overlay on the window's start line.  I think it's
pretty useful, as it helps in keeping context when working e.g. in
defuns with many lines of code -- all without having to scroll up and
down.

If this extension sounds useful to be included for some of you as well,
I would be very happy to get some feedback.  There for sure are things
to improve.  (It actually works quite well with the use cases I tried.)

To try it out:

- emacs -Q
- In scratch buffer, enter some long lisp expressions that overlap the
  window.
- (setq show-paren-style 'prelude)
- (show-paren-mode)

Note 1: There may already be such functionality available that I'm not
aware of.  Please let me know if that is the case.

Note 2: When the fade-in line contains multiple opening parens, the
matching one is highlighted.

Note 3: I'm not too sure about the colors to choose for the new
`show-paren-prelude' face.

Any feedback is welcome.

Thanks,
Alain