fix off-by-one in writePNG() method
fix off-by-one in writePNG() method

houqp/koreader-base,koreader/koreader-base,apletnev/koreader-base,Frenzie/koreader-base,koreader/koreader-base,NiLuJe/koreader-base,NiLuJe/koreader-base,Frenzie/koreader-base,NiLuJe/koreader-base,frankyifei/koreader-base,Frenzie/koreader-base,apletnev/koreader-base,frankyifei/koreader-base,apletnev/koreader-base,frankyifei/koreader-base,houqp/koreader-base,Frenzie/koreader-base,koreader/koreader-base,Hzj-jie/koreader-base,houqp/koreader-base,koreader/koreader-base,Hzj-jie/koreader-base,Hzj-jie/koreader-base,frankyifei/koreader-base,apletnev/koreader-base,NiLuJe/koreader-base,Hzj-jie/koreader-base,houqp/koreader-base