added gamma setting to drawcontext
added gamma setting to drawcontext

usage in Lua:
dc:setGamma(1.5)
print(dc:getGamma())

set it to a negative value to disable gamma correction
(since gamma correction will always imply calculations
over the full buffer data).

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