bug#65042: 30.0.50; alpha-background doesn't effect background of stipple in emacs 30.0.50 pgtk

[Felix <felix.dick@web.de>]

The background of stipple stays opaque with alpha-background parameter
set for transparency.
This came up in the use of the new package 'indent-bars'.
The github issue: https://github.com/jdtsmith/indent-bars/issues/8
I tested this with 'emacs -Q' only enabling the needed packages (compat
and indent-bars).


In GNU Emacs 30.0.50 (build 12, x86_64-pc-linux-gnu, GTK+ Version
 3.24.38, cairo version 1.17.8) of 2023-08-03 built on
 felix-lifebooka531
Repository revision: 588a0363d9a3ce6d678618ad545f7a8f9af27880
Repository branch: makepkg
System Description: Arch Linux

Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --mandir=/usr/share/man --with-gameuser=:games
 --with-modules --without-libotf --without-m17n-flt --without-gconf
 --enable-link-time-optimization --with-native-compilation=yes
 --with-xinput2 --with-pgtk --without-xaw3d --with-cairo-xcb
 --with-sound=no --with-xwidgets --with-tree-sitter --without-gpm
 --without-compress-install
 '--program-transform-name=s/\([ec]tags\)/\1.emacs/'
 'CFLAGS=-march=native -mtune=generic -O3 -pipe -fno-plt -fexceptions
 -Wp,-D_FORTIFY_SOURCE=2 -Wformat -Werror=format-security
 -fstack-clash-protection -fcf-protection'
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LCMS2 LIBSYSTEMD LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER PGTK
PNG RSVG SECCOMP SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS TREE_SITTER
WEBP XIM XWIDGETS GTK3 ZLIB

Important settings:
  value of $LANG: de_DE.UTF-8
  locale-coding-system: utf-8