Speedup of display of long and truncated lines

[Eli Zaretskii <eliz@gnu.org>]

I've installed a few changes intended to speed up redisplay of buffers
with long lines when lines are truncated.  This is WIP.  With the
changes I installed, navigation via C-f/C-b should be much faster, and
likewise C-e and C-a (although C-a still has one problem I see that
needs to be fixed).  Vertical cursor motion commands (C-n/C-p) are
still slow and need to be sped up; that's on my TODO.

I didn't yet look at C-v/M-v, mainly because I don't have examples of
files with long lines that have enough long lines to justify movement
by window-full.  (I could, of course, produce such files myself, but
the question is: do such files exist in Real Life, enough to make the
scrolling commands important in these cases?  Examples of such files,
if they exist, are welcome.)

I'm sure these changes break some use cases, or even cause crashes.
Reports of such issues are therefore welcome, as are examples of files
with long lines which exhibit slow redisplay after these changes
(under line-truncation).

Thanks in advance, and happy hacking!