Fixed texture pack loading, singleplayer launching, and several other bugs.

- Rewrote TextureManager.load() for better readability/reliability/performance. This fixed most texture-pack related problems.
- Fixed letter spacing in custom fonts. Support for high-resolution fonts still needs a bit of work.
- Fixed letter spacing for characters 0-31. This effectively implements "EmoteFix" CPE, which is now marked as supported.
- Fixed some incorrect CPE-related warnings in PacketHandler.
- Fixed singleplayer not launching properly, due to a bug in command-line param parsing.