Add adventure world-specific battle and menu background support

[vanja-ivancevic]

• Allow adventure worlds to provide custom battle backgrounds by placing files in their skin/ directory, using the same pattern that SoundSystem uses to allow custom music.
• The texture loading order for adventure backgrounds (ADV_BG_*) would be:
  1. Adventure world skin directory (e.g., Shandalar Old Border/skin/adv_bg_swamp.jpg)
  2. Common adventure skin directory (common/skin/)
  3. Default skin directory (fallback)

[Jetz72]

Does the background correctly update if you change planes without restarting the application? I think load is only invoked when the texture first appears, and I don't know if it's unloaded after that point.

[vanja-ivancevic]

Does the background correctly update if you change planes without restarting the application? I think load is only invoked when the texture first appears, and I don't know if it's unloaded after that point.

It doesn't automatically update without restarting. However, changing planes prompts the user to restart anyways.

[Jetz72]

I'd like to think we'll eventually get rid of the need to restart to change planes. I don't like adding to the list of problems that will need to be identified and solved in order to achieve that. Could just use a static method to clear out all the adventure background textures and invoke it when switching planes. (Which would be another use case for that EnumSet just sayin' 😝)

[vanja-ivancevic]

Great point. Added invalidateAdventureTextures() which resets the cached textures when switching planes - called alongside setAdventureDirectory() in all four places. Also switched to using an EnumSet as you suggested. Would this work?