bug#43209: 27.1; rectangle-mark-mode does not activate region if transient mark mode disabled

[Allen Li <darkfeline@felesatra.moe>]

rectangle-mark-mode does not activate region if transient mark mode is
disabled.

Steps:

1. emacs -Q
2. M-x transient-mark-mode RET
3. C-x SPC

Expected:

Region is activated temporarily.

Actual:

Region is not activated.  Note that rectagle-mark-mode is still
partially enabled.  For example, C-x C-x moves point according to the
rectangular region, but you cannot move point beyond the end of the
line.

I propose making rectangle-mark-mode activate the region even if
transient-mark-mode is disabled for a number of reasons.

I think it's a reasonable interpretation of the docstring:

Activates the region if needed.  Only lasts until the region is
deactivated.

rectangle-mark-mode is transient like having an active region and unlike
having a mark.  That is to say, someone who uses the mark with
transient-mark-mode off may not want to operate on the region and thus
not want to activate the region (e.g., when using the mark to jump to
later), but there is no use case for rectangle-mark-mode that does not
involve operating on the region.

Also since rectangle-mark-mode is transient, it makes sense to couple it
with region activation as the active region is also transient with the
same lifespan.

In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.22)
 of 2020-08-13 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
System Description: Arch Linux