fix: gltfio createInstance() overwrites shared morph target upload state

[mp0rta]

fix #9914

Summary

Calling createInstance() on an asset with morph targets can overwrite morph-target upload state that is shared through MeshCache, causing ResourceLoader to upload morph position data to the wrong MorphTargetBuffer.
After a second instance is created, the primary instance can stop receiving the correct morph updates.

Fix

Detect whether morph setup has already been performed for the shared Primitive.

• For the first instance, keep the existing behavior and populate the preallocated morph BufferSlots.
• For additional instances, preserve the existing shared state and append new morph BufferSlots that point to the new instance's MorphTargetBuffer.

This fixes morph position uploads for the non-extended (ResourceInfo) path and prevents later createInstance() calls from breaking the primary instance.

Known Limitations

• Morph tangents: computeTangents() still reads MorphTargetBuffer from the shared Primitive, so additional instances with lit morph targets may still get incorrect tangent frames. This patch only fixes morph position uploads.
• ResourceInfoExtended: The extended loader path is not addressed by this patch. Supporting multi-instance morph targets there would require carrying per-instance CPU-side morph data (targetData.positions, targetData.tbn) into the upload stage.

Tests

• VRM avatar with facial blend shapes + outline instance on Pixel 6a (Android 16, OpenGL ES)
• createInstance() no longer breaks morph targets on the primary instance
• Morph position data is uploaded to each instance's MorphTargetBuffer independently

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.