Lazy image converters

[Evgeny Zajcev <lg.zevlg@gmail.com>]

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It is a nice feature in Emacs 27 - 'image-convert' type for the images.
To convert unsupported images to format Emacs can understand.

Normally image creation (with 'create-image') is a very lightweight
process, Emacs don't read the files or do any heavy things.  All heavy
things are done in redisplay, so you could create and insert 1000 images in
the buffer with a couple of them visible and this will be fast.

However, with 'image-convert' images, this won't work, because heavy things
are done in the 'create-image', not in redisplay.  So Emacs will convert
1000 images at once.

What if we have a lazy image, so its FILE-OR-DATA is calculated at
redisplay time.  'image-convert' could utilize this.  One of the approaches
to lazy images is to allow FILE-OR-DATA to be a function returning actual
file or data.  Redisplay could call this func and substitute the value of
FILE-OR-DATA in image spec with the results, kind of caching the results.

What do you think?

Thanks

-- 
lg

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