Fix #3647 [backport to 1.4.x]

This fixes #3647, for me on the tkagg backend.
What still is strange is that the bug does not occur for e.g. cairo backend.
What about making the combined dict a local variable, like

```
bbox_params = dict(self._bbox, clip_box=self.clipbox, ...
```

I'm somehow against syncing some state, that is internal and exactly determined by some other state.