meshoptによる圧縮の有効化

[nokonoko1203]

Close #0

What I did

GLB出力に EXT_meshopt_compression による頂点バッファ圧縮を追加した。

主な変更:

• --meshopt CLIオプションの追加。--quantize との併用も可能
• cesiumtiles-gltf-json に EXT_meshopt_compression 関連の型定義を追加（ExtMeshoptCompression, MeshoptCompressionMode, MeshoptCompressionFilter
  等）
• pcd-exporter/gltf.rs をリファクタリング
• VoxelDecimator にMortonコードによる空間ソートを追加し、meshoptの圧縮効率を向上
• gzip圧縮を flate2 に変更

Notes

[sentry]

Bug: For an empty point cloud without meshopt, an invalid GLTF is created where a BufferView references a non-existent buffer, because the buffers array is empty.
_{Severity: HIGH}

Suggested Fix

Conditionally create the buffer_views array only when the buffers array is not empty. For example, wrap the buffer_views creation in an if uncompressed_len > 0 block, similar to how the buffers array is created. This will ensure that no BufferView is created when there are no buffers, resulting in a valid GLTF for empty point clouds.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: pcd-exporter/src/gltf.rs#L273-L280

Potential issue: When generating a GLB file for an empty point cloud without meshopt
compression, the code produces an invalid GLTF structure. If the `uncompressed_len` is
0, an empty `buffers` array is created. However, a `BufferView` is still generated, and
its `buffer` field defaults to `0`, causing it to reference a buffer that does not exist
in the empty array. This can occur when the decimation process results in an empty point
cloud, which is a possible production scenario. The resulting invalid GLB file violates
the glTF specification and can cause loading failures or crashes in client applications.

_{Did we get this right? 👍 / 👎 to inform future reviews.}