fix operator precedence #95
fix operator precedence #95

This explains why our dimmed areas had vertical stripes and improves
quality

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