zink: implement indirect buffer indexing

zink: implement indirect buffer indexing

this compacts all buffers in the shader into an array that can be
used in a single descriptor, thus handling the case of indirect indexing
while also turning constant indexing into indirect (with const offsets)
since there's no sane way to distinguish

a "proper" implementation of this would be to skip gl_nir_lower_buffers
and nir_lower_explicit_io altogether and retain the derefs, but that would
require a ton of legwork of other nir passes which only operate on the
explicit io intrinsics

Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15906>