bug#28855: 26.0.90; display-line-numbers-mode does not respect (line|wrap)-prefix '(space :align-to N) text property

[Paul Rankin]

display-line-numbers-mode does not respect line-prefix or wrap-prefix text 
properties when set as '(space :align-to N).

To reproduce:

1. emacs -Q
3. type some text in *scratch* buffer
2. M-: (put-text-property (line-beginning-position) (line-end-position) 
'line-prefix '(space :align-to 2))
3. M-x display-line-numbers-mode

Expected results:

Line numbers should be displayed before line-prefix (and/or wrap-prefix) space 
of 2 columns.

Actual results:

Line numbers eat the line-prefix and wrap-prefix space, effectively aligning 
the text back to 0.

Configuration:

GNU Emacs 26.0.90 (build 1, x86_64-apple-darwin17.0.0) of 2017-10-15
macOS 10.13 (17A405)

MacBook Pro (Retina, 15-inch, Mid 2015)
2.2 GHz Intel Core i7
16 GB 1600 MHz DDR3
Intel Iris Pro 1536 MB