Add legacy-x11 build option, and move DRI2 protocol support behind it

This is another attempt to excise DRI2 protocol support from the build. A new list option legacy-x11 is introduced, and the dri2 feature for it corresponds to X11 protocol support for the DRI2 extension. We no longer build that support by default, removing it incrementally from GLX, EGL, and the video frontends. The result no longer links against libxcb-dri2 or (for GLX) defines any xlib binding to DRI2.

This includes !29910 (merged) to avoid any merge conflicts in egl/x11.