This is a metabug tracking adding accelerated compositing support to the GTK+ port.
Heads up: I'm reworking some stuff in TextureMapper, which might affect some of these efforts. One of those is removing the TextureMapperQt backend and putting a cross-platform ImageBuffer-backed implementation instead, which might make the Cairo fallback unnecessary!
Since the GTK+ backends are not yet tested by the bots and contain a lot of notImplemented(), it's hard for me to keep track of whether I break something... Please stay tuned to avoid surprises at https://bugs.webkit.org/show_bug.cgi?id=75918 :)
I have a preliminary TextureMapperGL patch here that I'll try to upload. Hopefully our work doesn't clash. I'm looking forward to the ImageBuffer backend!
Closing this now, because it's essentially implemented completely. I think that AC via clutter could have its own metabug, if one isn't open yet.