bug#47895: 28.0.50; Emacs should only animate images that are visible

[Lars Ingebrigtsen]

Emacs is generally slow about calculating animated images, and that
should be fixed (there's a separate bug report for that), but there's
one thing Emacs could be doing:

Not animating images that are off screen.

I think most web browsers do this -- to avoid using too much CPU, which
is the same problem we have in Emacs: Opening a web page with many large
animated GIFs usually makes Emacs very slow.


In GNU Emacs 28.0.50 (build 78, x86_64-pc-linux-gnu, GTK+ Version 3.24.24, 
cairo version 1.16.0)
 of 2021-04-11 built on xo
Repository revision: 751e801f90339480ea43fc2237fc45c8eb39bd6f
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12010000
System Description: Debian GNU/Linux bullseye/sid


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(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no