After updating from master I wanted to try the new
mouse-drag-region-rectangle command but it didn't work with my init file,
and I found the reason was I have transient-mark-mode customized to
nil.  To reproduce:

0. emacs -Q (transient-mark-mode enabled)
1. Type `C-M-mouse-1', drag the mouse and a retangle gets selected.
2. M-x transient-mark-mode (now disabled)
3. Type `C-M-mouse-1', drag the mouse and no retangle gets selected.

I found no indication in NEWS, the Emacs manual or the doc string of
mouse-drag-region-rectangle that it is not supposed to work with
transient-mark-mode disabled, so I assume this is a bug.  This patch
makes it work with t-m-m disabled: