vk/meta: Save and restore the old bindings pointer

vk/meta: Save and restore the old bindings pointer

If we don't do this then recursive meta is completely broken.  What happens
is that the outer meta call may change the bindings pointer and the inner
meta call will change it again and, when it exits set it back to the
default.  However, the outer meta call may be relying on it being left
alone so it uses the non-meta descriptor sets instead of its own.