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fix flutter#111370

According to 

https://github.com/flutter/flutter/blob/5b47fef613e2d74ae44e1e22c2a3495294db180e/packages/flutter/lib/src/rendering/box.dart#L1259
: A [RenderBox] that uses methods on [PaintingContext] that introduce
new
/// layers should override the [alwaysNeedsCompositing] getter and set
it to
/// true.

set [alwaysNeedsCompositing] to true when RenderParagraph introduces
LeaderLayer for selection handles.



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- [ ] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] All existing and new tests are passing.

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