FFI framebuffer implementation adaptions

FFI interface to the framebuffer allows for a few simplifications. We can now use the blitbuffer API rotation interface to care for screen rotation. Thus, we can completely get rid of shadow framebuffers. Screen.bb is now fb.bb. In order to work properly with the rotation modes, Screen.width and Screen.height are gone now. Use Screen:getWidth() and Screen:getHeight() instead (they existed before).

The changes to koreader/koreader-base have to be applied first.