Add picture to explain quest size

[christopho]

In Quest data files format specification > Quest properties files, we explain what is the quest size and how it can has different value.
Pictures would help to understand

• that this is the logical scene size before scaling it to the window,
• that allowing a range of quest sizes can improve portability by replacing black bars with actual content,
• but that quest makers must then take care that their maps and their menus should adapt to the quest size range.