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.
replace Screen.width/Screen.height with accessor methods

Screen.width and Screen.height are now gone, use the methods
getWidth() and getHeight() to get the relevant data. They will
take blitbuffer/framebuffer rotation into account.
Merge remote-tracking branch 'upstream/master' into native-framebuffer
update koreader-base

This will need https://github.com/koreader/koreader-base/pull/96 to be applied first
Merge pull request #370 from hwhw/native-framebuffer

FFI framebuffer implementation adaptions