nir: add has_txs flag

Some nir lowerings might need to know if txs is supported by
the backend.

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8898>

src/compiler/nir/nir.h

@@ -3387,6 +3387,10 @@ typedef struct nir_shader_compiler_options {
     * iadd(x, ineg(y)). If true, driver should call nir_opt_algebraic_late(). */
    bool has_isub;
 
+   /** Backend supports txs, if not nir_lower_tex(..) uses txs-free variants
+    * for rect texture lowering. */
+   bool has_txs;
+
    /* Whether to generate only scoped_barrier intrinsics instead of the set of
     * memory and control barrier intrinsics based on GLSL.
     */