bug#50579: 28.0.50; [feature/pgtk] x-stretch-cursor not respected

[Illia Ostapyshyn]

When using the box cursor-type the cursor is stretched over the glyph
under it, acting as if the value of x-stretch-cursor were non-nil. The
value of that variable doesn't seem to have any effect.

I'm attaching a screenshot of emacs -Q with a cursor over a tab
character.

In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.20, cairo 
version 1.16.0)
 of 2021-09-14 built on lab-pc42
Repository revision: 4c49ec7f865bdad1629d2f125f71f4e506b258f2
Repository branch: feature/pgtk
Windowing system distributor 'System Description: Ubuntu 20.04.3 LTS

Configured using:
 'configure --with-native-compilation --with-pgtk
 --prefix=/home/user/.local 'CFLAGS=-O2 -march=native
 -fomit-frame-pointer''

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LIBSELINUX MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER PGTK PNG SECCOMP
SOUND THREADS TIFF TOOLKIT_SCROLL_BARS XIM GTK3 ZLIB

pgtk-cursor.png
Description: pgtk cursor stretching