Compatible GLX swap event fixes

[Jesse Barnes]

This set is compatible with existing code bases since it adds a new
struct to the glx and dri2 proto files instead of renaming existing
fields.

New clients with old servers will see larger, but still broken, SBC
values.  Old clients with new servers will see the same bad values they
see today.  And new clients with new servers will see correct SBC
values (yay).

I won't push until I get at least a couple of reviewed-by and at least
one tested-by from people.

Thanks,
-- 
Jesse Barnes, Intel Open Source Technology Center