Redisplay bug for 21.1

[Jason Evans]

I've seen many (>10) redisplay bugs when viewing the same buffer in two
windows.  The following URL shows one such case.

http://www.canonware.com/~jasone/emacs_bug.png

The bottom window is correct.

The operations that led up to this were:

Yank at line 333:

cw_nxo_t           *ostack, *tstack, *nxo, *tnxo;

Mark beginning of next line.
Move down one line.
Kill region.

Making additional changes to the buffer has tended to fix these glitches,
which is why I've only caught them out of the corner of my eye and
destroyed the evidence before realizing there was a problem.

Thanks,
Jason