mesa: add a pipe_context pointer to gl context

This will be used to move more code over

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14133>

src/mesa/main/mtypes.h

@@ -5574,6 +5574,7 @@ struct gl_context
    /*@{*/
    struct vbo_context vbo_context;
    struct st_context *st;
+   struct pipe_context *pipe;
    /*@}*/
 
    /**

src/mesa/state_tracker/st_context.c

@@ -981,6 +981,7 @@ st_create_context(gl_api api, struct pipe_context *pipe,
       return NULL;
    }
 
+   ctx->pipe = pipe;
    st_debug_init();
 
    if (pipe->screen->get_disk_shader_cache)