EnvMapAppearance fix, third person mode no longer limited by -noClip, textures now freed properly.

- Receiving EnvSetMapAppearance during the level-loading process no longer crashes the client.
- All EnvSetMapAppearance fields are now handled correctly.
- Disabling server-side texture packs no longer prevents servers from setting edgeBlock, sideBlock, and edgeLevel.
- Fixed third-person mode ability being affected by "-noClip" hack flag.
- Fixed default texture pack being saved to the settings file as "null".
- Fixed old textures never being freed when switching texture packs.
- Changed most texture names from string literals (e.g. load("/gui/gui.png")) to constants (e.g. load(Textures.GUI)).
- Texture pack selection GUI now shows "game broke" message with exception details if default texture failed to load, instead of suppressing errors and leaving the game in an inconsistent state.