VOXELFORMAT: add streaming OBJ export for large models

[Tudi]

Summary

• Adds a low-memory streaming OBJ export path that processes scene graph nodes one at a time with chunked 256^3 surface extraction
• Bypasses the merge-all-nodes path (sceneGraph.merge()) that allocates the full bounding box as a single RawVolume - prohibitive for large scenes (e.g. 1800+ nodes, ~7GB loaded would require ~160GB merge allocation)
• Enabled via voxformat_streamprocessobj cvar (default false, preserves existing behavior)
• Memory usage per chunk: ~25MB for extractor arrays plus mesh output, freed each iteration
• Writes directly to disk via SeekableWriteStream - no in-memory buffering of the full OBJ

Test plan

• Export a multi-node scene with voxformat_streamprocessobj false (default) - verify identical output to current behavior
• Export with voxformat_streamprocessobj true - verify correct OBJ output with all nodes properly positioned
• Export a large scene (1000+ nodes) with streaming enabled - verify memory stays bounded
• Verify voxformat_transform_mesh true correctly applies world transforms in streaming mode

[mgerhardy]

i would really prefer general solutions here and not per-format solutions were every other mesh based format could benefit from, too

maybe a generic streaming interface for the MeshFormat (not sure how that would work, because it would need some pre-production to get position locality to stream the triangles in the appropriate order (like the vertex cache on the gpu)

[Tudi]

voxformat OGT library accepts full scenes as input. Just one example. I hated the patch so much I canceled the PR myself. Not sure how many formats can work with stream processing/writing.
Maybe you have an idea ?

[mgerhardy]

for ogt_vox we have an issue here: jpaver/opengametools#80