bug#69837: 29.2; vtable-update-object only works in visible windows

[Adam Porter]

Hi,

I've discovered that `vtable-update-object' only works in buffers that 
are in visible windows.  When trying to update vtables in buffers that 
aren't, the object being updated or replaced fails to be found in the 
cache, apparently because `vtable--cache-key' uses `window-width' in its 
value (so maybe if, when the vtable buffer is not visible, the selected 
window happens to have the same width as the window in which the vtable 
buffer was previously displayed, it will work by chance).

In my case, the vtable is being updated in the background from a timer, 
so its buffer may or may not be visible.  It's preferable to call 
`vtable-update-object' to update two rows in it rather than reverting 
the whole table, because it can be relatively slow to revert the whole 
table when it's large.

I'm not sure of the best way to fix this.  Maybe the vtable's last-used 
window width could be cached and used for the cache key in case its 
buffer is not visible.  I guess that would risk displaying the vtable 
sub-optimally if the next time it's displayed its window width is 
different, but that might be worth it; in that case, the user could 
always revert the table if necessary.

I'm willing to work on a patch for this, but I'd appreciate any 
guidance, since I'm far from an expert on this library, and there are 
many nuances when it comes to buffers, windows, their attributes, etc.

Thanks,
Adam