added framework to check the involved pointers in blitbuffer operations
added framework to check the involved pointers in blitbuffer operations

blitbuffer operations in 4bpp can be a nasty experience, easy to
get the pointers wrong. so a macro can check them now.

Conflicts:

	blitbuffer.c

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