add alpha-pane based blitting and "adding" of colors
add alpha-pane based blitting and "adding" of colors

this allows to use the Alpha value in BB_8A and BB_RGB32 blitbuffer
layouts or Color8A and ColorRGB32 values, respectively.

addblitFrom() uses the new possibility, and there is alphablitFrom()
which allows to blit with awareness for full 8bit alpha.

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