GPM and mouse highlight

[Eli Zaretskii <eliz@gnu.org>]

A text terminal with a GPM mouse can support mouse highlight.
However, the implementation of this support in term.c uses static
variables to track the highlighted region.  Unless I'm missing
something, this means that this support cannot be available on more
than one terminal device at the same time, even though we have
multi-tty support for quite some time.

By contrast, the GUI sessions (and the MS-DOS port) have these
variables as part of the Display_Info structure and can track several
frames/devices independently.

Questions:

1) Is there some fundamental limitation of GPM that prevents it from
supporting more than a single device at a time?

2) If the answer to the previous question is NO, then should we add
the mouse-highlight related variables to struct tty_display_info on
Unix as well?

I'm asking because, as part of working on mouse highlight of
bidirectional text, I found out that there are 3 separate
implementations of mouse highlight: one for when we
HAVE_WINDOW_SYSTEM, another one for GPM, and yet another for MS-DOS.
Due to this, bugs that were fixed in the GUI version of this support
(e.g., bug#1220) are still there in the other 2 versions.  I'd like to
merge all 3 implementations into a single one.  (If nothing else, it
will save me from writing 3 very similar implementations of bidi-aware
mouse highlighting.)