remove redundancy: new framebuffer supports rotation, multi-bpp
remove redundancy: new framebuffer supports rotation, multi-bpp

We can get rid of shadow blitbuffers. Screen.bb is now fb.bb.
All bpp conversion is now done by the blitbuffer abstraction layer.
Rotation mechanism is adapted to this.

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