bug#31240: 26.1; mouse-save-then-kill does not kill rectangles

[Charles A. Roelli]

> From: Federico Tedin <federicotedin@gmail.com>
> Date: Tue, 25 Sep 2018 21:33:35 -0300
> 
> Charles, thanks for your input. I've created a new patch which fixes
> the handling of rectangular regions on both mouse-save-then-kill and
> mouse-drag-and-drop-region (it includes the changes made in the
> previous patch). I'm attaching it below.

Thanks!

I tried this to test the patch:

src/emacs -Q
M-x set-variable RET mouse-drag-and-drop-region RET t
With the mouse, drag from the "n" in "not saved" to the space after "C-x C-f" 
(in *scratch*)
C-x SPC
Now drag the region until the cursor is on the "a" of "and"

For some reason, the region is not moved, and it gets re-activated as
a normal region instead of a rectangular region.  Maybe I'm missing
something obvious.  I was able to get the dragging of a rectangular
region working sometimes, but not consistently.