egl: Implement EGL_KHR_context_flush_control

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Adam Jackson <ajax@redhat.com>

src/egl/main/egldisplay.h

@@ -112,6 +112,7 @@ struct _egl_extensions
 
    EGLBoolean KHR_cl_event2;
    EGLBoolean KHR_config_attribs;
+   EGLBoolean KHR_context_flush_control;
    EGLBoolean KHR_create_context;
    EGLBoolean KHR_fence_sync;
    EGLBoolean KHR_get_all_proc_addresses;