Fix partial refreshes on older Kobo devices
Fix partial refreshes on older Kobo devices

Fix #1146 (regression).

poire-z/koreader,chrox/koreader,ashhher3/koreader,Frenzie/koreader,koreader/koreader,apletnev/koreader,Markismus/koreader,koreader/koreader,mihailim/koreader,noname007/koreader,frankyifei/koreader,NiLuJe/koreader,Frenzie/koreader,NiLuJe/koreader,houqp/koreader,chihyang/koreader,mwoz123/koreader,robert00s/koreader,NickSavage/koreader,ashang/koreader,lgeek/koreader,pazos/koreader,Hzj-jie/koreader,poire-z/koreader