Certain animated images take an exceedingly long time (~minutes) to
encode to WEBP using method=6. Such a file was obtained, and it
encoded significantly faster using method=5 and somehow turned out
smaller than the result generated with method=6. We switch now to
use method=5 so we stop getting occasional massive increases in
CPU load when multiple users upload animations at the same time.
Additionally, a timeout was added to the ImageMagick subprocess just
in case there are even more pathological files in the future. Better
to error out than have to get someone to kill the process.