Add billboard capability

[kingjin94]

Add sprite drawing as a way to add billboards (2D text annotations that always face the camera). Allows to write text unto the scene.

The underlying changes to meshcat are proposed in this PR: meshcat-dev/meshcat#145

An example:

Code to recreate the screenshot:

import meshcat
import numpy as np
t = meshcat.geometry.BillboardObject("test444", 75, 16, 0.01)

# !pip install timor-python
from timor.utilities import prebuilt_robots  
r = prebuilt_robots.get_six_axis_assembly()
viz = r.robot.visualize()

viz.viewer["test"].set_object(t)
viz.viewer["test"].set_transform(np.asarray(((1, 0., 0., 1.), (0, 1, 0, 1),(0, 0, 1, 0),(0, 0, 0, 1.))))

# Alternative w/out robot
viz = meshcat.Visualizer()
viz.initViewer()
viz["test"].set_object(t)
viz["test"].set_transform(np.asarray(((1, 0., 0., 1.), (0, 1, 0, 1),(0, 0, 1, 0),(0, 0, 0, 1.))))