Apply JPEG encoding for better performance

[almarklein]

• [x] Implement JPEG encoding based on simplejpeg and Pillow.
• [x] Tests.
• [x] Add optional dependencies and dev dependencies.
• [x] Make widget use encoding.
• [x] Always end with a PNG.
• [x] Decrease throttle-time in JS a bit.
• [x] Document (and maybe rename?) new quality trait.

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This example, but with a canvas size of 800x600, renders 5 FPS before this PR. But with JPEG encoding it renders at 28 FPS.

This Vispy example, at 800x600 renders at 30+ FPS (20 FPS with lossless (PNG)),

[djhoese]

I was thinking about it more...is the "end with PNG" feature worth the complexity? Do you see a difference in the quality? I guess if it can be controlled then that isn't too big of a deal.

[almarklein]

is the "end with PNG" feature worth the complexity?

I think it is. The difference in quality would be barely noticeable with quality 90. But thanks to this addition:

• If you do right-click and save image, it's always the picture perfect version.
• You can lower the quality on slow connections and still see the high quality image when the scene is static.
• We can somewhat lower the default quality (with 80 you can see a difference if you look closely).

[djhoese]

If you do right-click and save image, it's always the picture perfect version.

Good point. Forgot about saving the image.

You can lower the quality on slow connections and still see the high quality image when the scene is static.

I like that idea. Kind of like video buffering but it is a more literal stream of images.

[almarklein]

Comments addressed.