imwayland: Clamp the surrounding string end correctly

imwayland: Clamp the surrounding string end correctly

In the paths where len > MAX_LEN and cursor/anchor are separated by
at least MAX_LEN from text edges, we were clamping the right end of
the surrounding string at MAX_LEN. Oops.

This end anchor may go as far as the string length, although just
up to len - MAX_LEN in real terms (due to the condition above that
caches cursor/anchor positions being near enough the text end).

Closes: https://gitlab.gnome.org/GNOME/gtk/-/issues/2565
(cherry-picked from commit d7fb15c822eabe22ffda19892153d69b0f4fc2f3)