docs: Mark GLX_ARB_context_flush_control done

Requires an unreleased X server, but from the client GLX side this is as
done as it gets.

Signed-off-by: Adam Jackson <ajax@redhat.com>

docs/features.txt

@@ -334,7 +334,7 @@ Khronos, ARB, and OES extensions that are not part of any OpenGL or OpenGL ES ve
   GL_OES_texture_half_float_linear                      DONE (i965, r300, r600, radeonsi, nv30, nv50, nvc0, softpipe, llvmpipe)
   GL_OES_texture_view                                   not started - based on GL_ARB_texture_view
   GL_OES_viewport_array                                 DONE (i965, nvc0, radeonsi)
-  GLX_ARB_context_flush_control                         not started
+  GLX_ARB_context_flush_control                         DONE
   GLX_ARB_robustness_application_isolation              not started
   GLX_ARB_robustness_share_group_isolation              not started