Dispose managed material plugins on feature destruction

[RaananW]

Ensure proper disposal of managed material plugins when the feature is destroyed to prevent memory leaks.

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

Snapshot stored with reference name:
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https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/16439/merge/index.html

To test a playground add it to the URL, for example:

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[bjsplat]

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[bjsplat]

WebGL2 visualization test reporter:

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