properly set up shadow buffer's pitch and bug fix on eink update

previously, counters never reach 0!
get correct screen width in Screen.lua
add more comments
Merge branch 'new_ui_code' of github.com:hwhw/kindlepdfviewer into new_ui_code
Merge pull request #750 from houqp/new_ui_code

bug fix for einkfb.c and screen.lua