v3dv/pipeline: track if texture is shadow

v3dv/pipeline: track if texture is shadow

To be used to decide the texture return size. We add it on the
descriptor map because it is the easier place to do so. As we are
lowering the texture accesses we can check instr->is_shadow at that
point. It is true that it is somewhat odd, as so far the descriptor
map was general-descriptor info, but is_shadow is only for
textures. But it doesn't make sense to make an effort now, as it is
possible that we would get more descriptor-specific info on the map on
the future. We can revisit that later.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6766>