compiler/types: Add a texture type

compiler/types: Add a texture type

This is separate from images and samplers.  It's a texture (not a
storage image) without a sampler.  We also add C-visible helpers to
convert between sampler and image types.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13389>