nir: Add a lowering pass for YUV textures

This lowers sampling from YUV textures to 1) one or more texture
instructions to sample each plane and 2) color space conversion to RGB.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/nir.h

@@ -2345,6 +2345,13 @@ typedef struct nir_lower_tex_options {
     */
    bool lower_rect;
 
+   /**
+    * If true, convert yuv to rgb.
+    */
+   unsigned lower_y_uv_external;
+   unsigned lower_y_u_v_external;
+   unsigned lower_yx_xuxv_external;
+
    /**
     * To emulate certain texture wrap modes, this can be used
     * to saturate the specified tex coord to [0.0, 1.0].  The