gallium,st/mesa: Add and set pipe_image_view::is_2d_view_of_3d

This will be useful for lavapipe since the Z-coordinate has to be
ignored for 2D view accesses.

Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31038>

src/gallium/include/pipe/p_state.h

@@ -534,6 +534,7 @@ struct pipe_image_view
          unsigned last_layer:16;      /**< last layer to use for array textures */
          unsigned level:8;            /**< mipmap level to use */
          bool single_layer_view;      /**< single layer view of array */
+         bool is_2d_view_of_3d;
       } tex;
       struct {
          unsigned offset;   /**< offset in bytes */

src/mesa/state_tracker/st_atom_image.c

@@ -114,6 +114,7 @@ st_convert_image(const struct st_context *st, const struct gl_image_unit *u,
          } else {
             img->u.tex.first_layer = u->_Layer;
             img->u.tex.last_layer = u->_Layer;
+            img->u.tex.is_2d_view_of_3d = true;
          }
       } else {
          img->u.tex.first_layer = u->_Layer + stObj->Attrib.MinLayer;