improve: lightweight preview to reduce network traffic (#733)

* To reduce bandwidth traffic in a remote environment, a lossy compression-based preview mode is provided for displaying simple visualizations in node-based widgets.

* Added 'preview=[image format]' option to the '/view' API.
* Updated node to use preview for displaying images as widgets.
* Excluded preview usage in the open image, save image, mask editor where the original data is required.

* Made preview_format parameterizable for extensibility.

* default preview format changed: jpeg -> webp

* Support advanced preview_format option.
- grayscale option for visual debugging
- quality option for aggressive reducing

L?;format;quality?

ex)
jpeg => rgb, jpeg, quality 90
L;webp;80 => grayscale, webp, quality 80
L;png => grayscale, png, quality 90
webp;50 => rgb, webp, quality 50

* move comment

* * add settings for preview_format
* default value is ''(= don't reencode)

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Co-authored-by: Lt.Dr.Data <lt.dr.data@gmail.com>