Write directly to GLB from TGeometries

[errissa]

The following script can be used to test this PR:

import open3d as o3d

# Load the Monkey and give it some material properties
d = o3d.data.MonkeyModel()
mesh = o3d.t.io.read_triangle_mesh(d.path)
img = o3d.t.io.read_image(d.path_map['albedo'])
roughness = o3d.t.io.read_image(d.path_map['roughness'])
metallic = o3d.t.io.read_image(d.path_map['metallic'])
normal = o3d.t.io.read_image(d.path_map['normal'])
mesh.material.material_name = 'defaultLit'
mesh.material.texture_maps['albedo'] = img
mesh.material.texture_maps['roughness'] = roughness
mesh.material.texture_maps['metallic'] = metallic
mesh.material.texture_maps['normal'] = normal
mesh.material.scalar_properties['metallic'] = 1.0
o3d.visualization.draw(mesh)

# Save mesh as GLB and reload it as a triangle mesh to check geometry
o3d.t.io.write_triangle_mesh('/home/renes/development/intel_work/test_scripts/monkey.glb', mesh)
mesh_test = o3d.t.io.read_triangle_mesh('monkey.glb')
o3d.visualization.draw(mesh_test)

# Read into legacy as model which supports loading materials and embedded texture from GLB
model = o3d.io.read_triangle_model('/home/renes/development/intel_work/test_scripts/monkey.glb')
o3d.visualization.draw(model)

NOTE: The resulting monkey.glb should also be importable in Blender and look like:

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This change is 

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[ssheorey]

Looks good. 2 minor comments and we can merge.