Emacs uses the PictStandardA4 picture format to display certain kinds of images. A "painfully slow" software fallback is not a problem, since the important thing is for the picture to be displayed in the first place.
So there are "users in the wild", and in any case the X.Org server should hold itself to its own protocol specifications. Thus, as an Emacs developer, I'd like to ask for: https://gitlab.freedesktop.org/xorg/xserver/-/commit/436fd7e8b4966c305ea9c43f3c14c2ca04c35539 to be reverted, or for a major version bump of the rendering extension. That commit came as a nasty surprise when I updated my local X server. Backwards incompatible changes should *NOT* be made out of the blue. No matter how many applications you look at, you will always miss one or two. And there may be private code, or code someone will write in the future. "All the major toolkits and libXft" do *NOT* cover every application in the wild, especially not Emacs, whose X Windows code predates all of those libraries and toolkits and refrains from relying on any them. I cannot stress that enough. Emacs has never needed to deal with X servers breaking backwards compatibility in the past and it would be a shame for that to start now. They should be made in accordance with what renderproto.txt says: Major versions changes can introduce incompatibilities in existing functionality, minor version changes introduce only backward compatible changes.