st/mesa: purge framebuffers when unbinding a context

This fixes pipe_surface "leaks".

Cc: 18.3 <mesa-stable@lists.freedesktop.org>
Acked-by: Timothy Arceri <tarceri@itsqueeze.com>

src/mesa/state_tracker/st_manager.c

@@ -1095,7 +1095,12 @@ st_api_make_current(struct st_api *stapi, struct st_context_iface *stctxi,
       st_framebuffers_purge(st);
    }
    else {
+      GET_CURRENT_CONTEXT(ctx);
+
       ret = _mesa_make_current(NULL, NULL, NULL);
+
+      if (ctx)
+         st_framebuffers_purge(ctx->st);
    }
 
    return ret;