Update Screen.cpp

Screen.cpp:
--------------
Screen buffer address MUST be divisible by 8.
If that's not the case, the display will be slightly shifted to the right.

The current nPDF version does not check this.
So when the additionnal screen buffer gets an address not divisible by 8, there will be a display problem once every 2 refreshes because of the double buffering.

Viewer.cpp:
--------------
Disable auto-next/previous page when scrolling to the very bottom/top of a page.
Changing the current page takes some time and, without scrolling bars, chances of triggering it by mistake are too high.

Main.cpp:
------------
Mostly rewritten to be more user-friendly by allowing simultaneous keypresses (downright, downleft...).