Make Widget.useblit robust against underlying canvas changes

Nobody has claimed this yet.

As noted in https://github.com/matplotlib/matplotlib/pull/30490#discussion_r2312635485 (and #30485, the original issue, also #25075), Widget.canvas, which is typically initialized as `self.useblit = useblit and canvas.supports_blit`, is fragile against changes in the underlying canvas (the most common case in practice being a temporarily switch from a canvas that supports blitting to one that doesn't, when saving to vector output).
@tacaswell wrote some code to handle that in #25085, but some further fixes were necessary in #30490 which uses a different approach; a unified way of handling the issue would be nice to have.

**Status:**

- [x] Button
- [x] _SelectorWidget
- [x] SpanSelector
- [x] ToolLineHandles
- [x] ToolHandles
- [x] RectangleSelector
- [x] LassoSelector
- [x] PolygonSelector

Not supported yet:

- [ ] CheckButtons
- [ ] RadioButtons
- [ ] Cursor
- [ ] MultiCursor
- [ ] Lasso

Start by reading the discussion in PR #30490, along with issues #30485 and #25075, and review the checked and unchecked widget list in this issue. Trace how Widget.canvas and useblit behave when the canvas changes during vector output; done means the remaining CheckButtons, RadioButtons, Cursor, MultiCursor, and Lasso cases are handled consistently.

Written by the indexing model from the issue text.