fix RGB16 getG() method
fix RGB16 getG() method

there was a typo that led to the return of wrong numbers when
asking a ColorRGB16 value for its (8bit) green value (getG()).
This led to very wrong results when blending colors.

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