SCUMM: Fix bug #6679 - INDY3 DOS-EGA: book of maps graphic glitch

This fixes the issue described in bug #6679 (purple box covering part of the screen while looking at the book of maps). A more detailed explanation of what goes on under the hood has been provided in the bugtracker:

http://sourceforge.net/p/scummvm/bugs/6679/

I've done some testing by playing through the game and this change doesn't seem to have introduced any side effects.