Improve performance of 3D scenes by sending multiple messages at once

[falkoschindler]

This PR experiments with sending all non-update messages at once to reduce the time for creating complex 3D scenes like discussed in #3009.

A demo with 10,000 boxes still takes a while:

with ui.scene() as scene:
    scene.move_camera(y=-6, z=7)
    for x in range(100):
        for y in range(100):
            scene.box().scale(0.09).move(x / 10 - 5, y / 10 - 5)

I'm not sure if sending tens of thousands of messages at once is a good idea or if we should create chunks. And it is unclear what the remaining bottleneck is. Should we optimize the Python code, the payload or the JavaScript code on the client? Maybe combining multiple steps for creating a 3D object (8 at the moment) into one function call would also be a reasonable improvement.

[kovalp]

Let's see if I manage to test this...

[kovalp]

I tested this change. It is a good move for a few reasons.

• the development version produces no flickering in Firefox and Chrome.
• the version in the main branch when viewed from Firefox produces a not completely filled square in many cases, while the version in the development branch always produced the expected field of boxes.

On the downside, the development version on Firefox results in a message "This page slows down the browser" with the button "Close".

[falkoschindler]

Yes, processing all these messages takes a while, which freezes the browser for a short while.
In PR #3022 I'm experimenting with another promising approach to speed up the initialization of large 3D scenes.