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>

src/compiler/glsl/ir_clone.cpp

@@ -354,6 +354,7 @@ ir_constant::clone(void *mem_ctx, struct hash_table *ht) const
    case GLSL_TYPE_UINT8:
    case GLSL_TYPE_INT8:
    case GLSL_TYPE_SAMPLER:
+   case GLSL_TYPE_TEXTURE:
    case GLSL_TYPE_IMAGE:
       return new(mem_ctx) ir_constant(this->type, &this->value);