Fix garbled ImageWidget cache issues
Fix garbled ImageWidget cache issues

Fixes #2805. Thanks to @poire-z, see https://github.com/koreader/koreader/issues/2805#issuecomment-296686695

Markismus/koreader,koreader/koreader,Frenzie/koreader,NiLuJe/koreader,lgeek/koreader,houqp/koreader,Hzj-jie/koreader,pazos/koreader,mihailim/koreader,robert00s/koreader,Frenzie/koreader,apletnev/koreader,koreader/koreader,poire-z/koreader,poire-z/koreader,NiLuJe/koreader,mwoz123/koreader