We only support the VariablePointersStorageBuffer feature for now,
which is the only one that is mandatory, and for which we seem to
be passing all the relevant tests already.

Exposing the optional VariablePointers feature would require that
we support non-constant indexing on UBO/SSBO first.

Relevant CTS tests:
dEQP-VK.*pointer*

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <mesa/mesa!11257>