Deprecate bbox and xy parameters to restore_region?

Nobody has claimed this yet.

### Describe the issue

**Summary**

`RendererAgg.restore_region` takes two optional kwargs, `bbox` and `xy`, which specify respectively a subregion and and offset for the restoring. These were introduced in b1c784d for manual animations where subregions of specific patterns would be blitted with certain offsets. This was demonstrated in a single example, which was removed in e547d6f, and never used anywhere else.

I would suggest deprecating and then getting rid of the feature: there doesn't seem to be much demand for it (no other use anywhere in the library or the examples); the kind of animation it allows is extremely specific, and likely it should be possible to build something similarly efficient using `FigureImage` (i.e., drawing a non-resampled array at a given position should really just involve setting the correspoding pixels in the bitmap) anyways.

Also, that means one less thing to implement in mplcairo for feature parity :)

Is there anyone opposed to such a removal?

Start at the RendererAgg.restore_region entry point and verify the reported uses of its bbox and xy parameters across the library and examples. Review the deprecation and removal expectations for this API; done means the obsolete feature has an agreed, complete removal path without breaking remaining uses or mplcairo parity.

Written by the indexing model from the issue text.