Add InstancedMesh support for efficient multi-environment rendering - #2

Summary

• Adds InstancedMesh support for rendering thousands of identical objects with a single draw call
• Enables efficient visualization of multiple robot trajectories (4000+) without performance degradation
• ~100x faster than creating individual mesh objects

Changes

Python (meshcat-python):

• geometry.py: New InstancedMesh class with set_matrix_at(), set_matrices(), set_color_at(), set_colors() methods
• commands.py: New SetInstanceMatrices and SetInstanceColors commands for efficient updates
• visualizer.py: New set_instance_matrices() and set_instance_colors() methods
• zmqserver.py: Added new command types

JavaScript (meshcat viewer):

• index.js: Handler for _InstancedMesh object type and set_instance_matrices/set_instance_colors commands
• Related viewer PR: Add InstancedMesh support for efficient multi-environment rendering meshcat#1

Usage Example

from meshcat.geometry import InstancedMesh, Box, MeshPhongMaterial

# Create 1000 instances with a single draw call
instanced = InstancedMesh(Box([0.1, 0.1, 0.1]), MeshPhongMaterial(), count=1000)

# Set transforms for each instance
for i in range(1000):
    matrix = np.eye(4)
    matrix[:3, 3] = [i * 0.2, 0, 0]
    instanced.set_matrix_at(i, matrix)

vis["robots"].set_object(instanced)

# Efficient updates - only sends transform data, not geometry
new_matrices = compute_new_transforms()
vis["robots"].set_instance_matrices(new_matrices)

# Optional per-instance colors
vis["robots"].set_instance_colors(color_array)

Test plan

• Run examples/instanced_mesh_simple.py - basic grid animation
• Run examples/instanced_mesh_demo.py - full feature demo with performance comparison

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