Allow customizing `CompactSubGeometry`.#66

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player-03 wants to merge 14 commits intoopenfl:masterfrom

player-03:VertexDefinition

Commits on Jul 20, 2025

Implement `VertexDefinition`, allowing users to change which attributes `CompactSubGeometry` includes.
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Commits on Jul 25, 2025

Clean up.
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Commits on Aug 22, 2025

Clone `CompactSubGeometry.definition`.
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Commits on Oct 9, 2025

Implement `VertexDefinition.toString()`.
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Commits on Jan 11, 2026

Account for `_vertexData` being null.
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