gallium: add pipe_draw_info::index::gl_bo

mesa/main will fill pipe_draw_info but it has no access to pipe_resources,
so we need gl_buffer_object here.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7679>

src/gallium/include/pipe/p_state.h

@@ -53,6 +53,7 @@
 extern "C" {
 #endif
 
+struct gl_buffer_object;
 
 /**
  * Implementation limits
@@ -778,6 +779,7 @@ struct pipe_draw_info
     */
    union {
       struct pipe_resource *resource;  /**< real buffer */
+      struct gl_buffer_object *gl_bo; /**< for the GL frontend, not passed to drivers */
       const void *user;  /**< pointer to a user buffer */
    } index;