sw_winsys: add displaytarget_create_mapped

this is for creating displaytargets from already-imported fds

Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27805>

src/gallium/include/frontend/sw_winsys.h

@@ -138,6 +138,14 @@ struct sw_winsys
    void 
    (*displaytarget_destroy)( struct sw_winsys *ws, 
                              struct sw_displaytarget *dt );
+
+   struct sw_displaytarget *
+   (*displaytarget_create_mapped)( struct sw_winsys *ws,
+                                   unsigned tex_usage,
+                                   enum pipe_format format,
+                                   unsigned width, unsigned height,
+                                   unsigned stride,
+                                   void *data );
 };