GLTFExporter: Fix skinIndex export for InterleavedBufferAttribute

[querielo]

This PR fixes an issue in GLTFExporter where skinIndex (JOINTS_0) attributes were being exported incorrectly if they were stored as an InterleavedBufferAttribute.

The issue can cause this exception:

When skinIndex data needed to be converted to UNSIGNED_SHORT (e.g. if the source was Uint32Array), the exporter was creating a new Uint16Array directly from the underlying attribute.array.

For InterleavedBufferAttribute, attribute.array refers to the entire shared buffer containing data for multiple attributes. This resulted in the exported skinIndex accessor containing mixed data from other interleaved attributes, ignoring the stride and offset.

[Copilot]

Pull request overview

This PR fixes a bug in GLTFExporter where skinIndex (JOINTS_0) attributes stored as InterleavedBufferAttribute were being exported incorrectly. The fix ensures that type conversion from Uint32Array/Int32Array to Uint16Array properly handles interleaved data by using element-by-element copying instead of direct array copying.

Key Changes:

• Modified the JOINTS_0 type conversion logic to use getComponent/setComponent for element-by-element copying
• Ensures correct handling of InterleavedBufferAttribute by respecting offset and stride during conversion

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[donmccurdy]

One minor comment suggested, otherwise this fix looks good to me -- thanks @querielo!

[donmccurdy]

To match the 'else if' block below, do you mind moving this loop to a new utility function like GLTFExporter.Utils.toUint16BufferAttribute( attribute )? Or the existing utility function could be extended and renamed as:

modifiedAttribute = GLTFExporter.Utils.toTypedBufferAttribute( attribute, Uint16Array );