Add back extension codes for Uint8Array and Int32Array

[jwnimmer-tri]

These were accidentally lost in the switch to msgpack-javascript (#132).

Closes #138.

+@SeanCurtis-TRI for both reviews, please.

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It's not really practical to test this with an HTML file in this repository. The problem is with the msgpack decoding, which we usually don't test here (we author JSON in tests, not msgpack).

Instead, I tested it manually with this Python program that uses meshcat-python:

import meshcat
import meshcat.geometry as g

vis = meshcat.Visualizer()
vis["path"].set_object(
    g.StlMeshGeometry.from_file("/home/jwnimmer/jwnimmer-tri/drake/geometry/test/quad_cube.stl"),
    g.MeshPhongMaterial())

import time
time.sleep(60)

With the master branch meshcat, it errors out with no cube (the browser console has an error message). With this branch, the browser shows the cube.

I didn't test the int32 decoder. (I can't find anywhere its used.) Hopefully the visual inspection and copy-paste from Uint32 is sufficient.

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This change is 

[SeanCurtis-TRI]

X2

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @jwnimmer-tri)

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src/index.js line 38 at r1 (raw file):

    const to_return = new Uint8Array(data.byteLength);
    let dataview = new DataView(data.buffer, data.byteOffset, data.byteLength);
    for (let i = 0; i < to_return.length; i++) {

BTW Is this a javascript thing instead of ++i?

Code quote:

i++

[jwnimmer-tri]

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @jwnimmer-tri)

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src/index.js line 38 at r1 (raw file):

Previously, SeanCurtis-TRI (Sean Curtis) wrote…

BTW Is this a javascript thing instead of ++i?

TBH, I have no idea. I quick peek at stack overflow suggests that in JS it doesn't matter. The index.js is currently divided with about half of the loops doing it each way.