This series of commits implement DMA-Buf sharing on top of PipeWire 0.3 new API.
The framebuffers are all created in one go; during the stream, they're recycled, and we blit the current onscreen framebuffer into the shared one; at last, when the stream is closed, all the framebuffers are deleted.
The result is, as expected, massive: with this MR applied, Mutter / GNOME Shell consumes almost no extra CPU beyond what it consumes regularly.