Add update_hfield, update_mesh, update_texture methods to Renderer class

[Wheeeeeeeeels]

Summary

Add three new methods to the Renderer class that allow uploading modified asset data to the GPU after the model has been loaded:

• update_hfield(hfieldid): Uploads height field data
• update_mesh(meshid): Uploads mesh vertex data
• update_texture(texid): Uploads texture data

This enables dynamic modification of terrain, meshes, and textures without recreating the entire renderer.

Fixes #1410

Implementation

These methods wrap the corresponding mjr_upload* C functions with proper validation and GL context handling. Each method:

• Checks if renderer is still open (raises RuntimeError if closed)
• Validates the ID is within valid range (raises ValueError if not)
• Makes the GL context current before calling the upload function

Example Usage

import mujoco

model = mujoco.MjModel.from_xml_path("terrain.xml")
data = mujoco.MjData(model)

with mujoco.Renderer(model, 480, 640) as renderer:
    # Modify height field data
    model.hfield_data[...] = new_terrain_data
    renderer.update_hfield(0)  # Upload to GPU
    
    renderer.update_scene(data)
    pixels = renderer.render()

Tests

Added 9 unit tests covering:

• Successful update operations for each asset type
• Out-of-range ID validation (negative and >= max)
• Post-close behavior (RuntimeError)

All 15 tests pass:

python/mujoco/renderer_test.py ... 15 passed in 5.14s

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