This patch replaces my previous borders-respect-alpha-background frame
parameter with an alpha-borders frame parameter, which lets users set the
alpha of borders to whatever they desire.

Most of the changes are just changing boolean respect_alpha_background
parameters to double alpha parameters. respect_alpha_background=true gets
replaced with f->alpha_background, and respect_alpha_background=false gets
replaced with 1.0 (opaque).

I have tested both the GTK3 and PGTK builds.

One detail I noticed (prior to this patch) is that on X, internal borders
are opaque. However, on PGTK, internal borders take on the alpha-background
opacity. Is this intentional? If not, I'm happy to fix it.

Best,
Garklein

In GNU Emacs 30.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.49,
cairo version 1.18.2)
Windowing system distributor 'The X.Org Foundation', version 11.0.12101016
System Description: NixOS 25.11 (Xantusia)

Configured using:
 'configure
 --prefix=/nix/store/xkc0y1ycq86fs24fipcbzkph2acwsjnz-emacs-gtk3-30.1
 --disable-build-details --with-modules --with-x-toolkit=gtk3
 --with-cairo --without-xft --with-compress-install
 --with-toolkit-scroll-bars --with-native-compilation
 --without-imagemagick --with-mailutils --without-small-ja-dic
 --with-tree-sitter --with-xinput2 --without-xwidgets --with-dbus
 --with-selinux'