Stop allocating visual IDs with FakeClientID()
It's not wrong, not exactly, but it means we end up with a big range of unallocated XIDs. They're different namespaces, they should probably use different allocators.
GLXFBConfigs also use FakeClientID
as an allocator, but the GLX spec does in fact call them XIDs, so maybe leave that alone.
Blocks: ajax/xserver#2