Menu backgrounds

[FolkertVanVerseveld]

The backgrounds use a custom text file format which contains multiple
reference IDs to shp/slp files. In order to properly render backgrounds
we have to:

• reverse engineer custom background file format
  • determine dimensions
  • extract palette
• reverse engineer rendering technique
• extend draw menu/rendering subsystem

The dimensions are probably hardcoded, since all supported fullscreen
resolutions are 640x480, 800x600, 1024x768 and 1280x1024 were 800x600 is
the best fit and other dimensions just stretch the background to make it
fit.

The rendering process is probably just blitting the background picture,
but we have to reverse engineer this to make sure that's all that's
required.

[FolkertVanVerseveld]

The aoesetup.exe program does not seem to do any post processing after
loading the RT_BITMAP resources. empires.exe probably doesn't do this
either, but we have to verify this.

The setup main screen and its buttons are loaded and displayed
correctly. Transparent pixels are not handled yet.

[FolkertVanVerseveld]

The game now properly renders background menus! This milestone means a
lot to me, because it took many months to reverse-engineer the whole
process. Finally, we can close this issue.