fix small bug, add fast path for alpha blitting with values 0, 0xFF
fix small bug, add fast path for alpha blitting with values 0, 0xFF

The default alpha value returned by ColorNN:getAlpha() is 0xFF now.
Also, add fast path (avoid complex calculation) for alpha-blitting
with alpha=0xFF and alpha=0.

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