Fixes https://github.com/flutter/flutter/issues/132788

The SliverGeometry was not set properly for SliverMainAxisGroup. Omitting hasVisualOverflow affected the Viewport's choice to apply a clip, leading to the sliver being rendered outside of the bounds of the viewport.

This PR changes the paint functions for SliverMainAxisGroup and SliverCrossAxisGroup so that only visible slivers are painted.

Fixes https://github.com/flutter/flutter/issues/129214.

This widget implements the ability to place slivers one after another in
a single ScrollView in a way that all child slivers are drawn within the
bounds of the group itself (i.e. SliverPersistentHeaders aren't drawn
outside of the scroll extent provided by all of the child slivers). The
design document for SliverMainAxisGroup can be found
[here](https://docs.google.com/document/d/1e2bdLSYV_Dq2h8aHpF8mda67aOmZocPiMyjCcTTZhTg/edit?resourcekey=0-Xj2X2XA3CAFae22Sv3hAiA).

Fixes https://github.com/flutter/flutter/issues/33137.

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we expect every widget to implement].
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above.
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---------
Co-authored-by: Kate Lovett <katelovett@google.com>

This PR fixes `SliverCrossAxisGroup` so that once we calculate the total `scrollExtent` of the `SliverCrossAxisGroup`, we ensure that any child sliver is not painted outside of the "scroll extent" of the sliver group.

https://github.com/flutter/flutter/assets/2004742/670dc6f9-a3c6-4bcc-85d3-576cf7f33c6a

https://github.com/flutter/flutter/assets/2004742/0c9cd951-c133-4a8b-9e5e-89d0a69a4f59

Fixes #126958.
Fixes #126957.

This widget implements the ability to place slivers side by side in a single ScrollView so that they scroll together. The design document for `SliverCrossAxisGroup` can be found [here](https://docs.google.com/document/d/1e2bdLSYV_Dq2h8aHpF8mda67aOmZocPiMyjCcTTZhTg/edit?resourcekey=0-Xj2X2XA3CAFae22Sv3hAiA).

Fixes #56756.