bug#56683: 29.0.50; long lines fix doesn't work correctly when lines are truncated

[Gregory Heytings]

By the way, we should also decide what to do when the user enables 
truncate-lines in a buffer with "really too long" lines.  With 
DISP_INFINITY == 10000000 (its current value), and with a character pixel 
width of 20 (a not uncommon value nowadays with HiDPI screens), lines 
longer than 500000 (single-width) characters will not be displayed 
correctly.  With a larger character pixel width, or with characters 
occupying more than one column, that would be even less.

Given:

1. that there has not been a single bug report about the fact that 
truncate-line does not work correctly with "really too long" lines in the 
last 20 years,

2. that the limit above, caused by DISP_INFINITY, is not much more than 
10000, the default value for long-line-threshold, and

3. the remaining slowdowns caused by truncate-lines,

I'd be in favor of simply disabling truncate-lines when 
long_line_optimizations_p is set.