Feature: M2 batch cleanup

[fallenoak]

These changes are being done in preparation for the more significant changes relating to proper shaders for Wrath of the Lich King content.

Changes

• Renamed texture units to batches (a more conventional term, and one that matches debug strings in the client)
• Synced attribute name changes in parsed M2 and skin data against Blizzardry
• Migrated batch definition logic into a dedicated batch manager to reduce clutter in the root M2 class
• Switched to single transparency animation per batch based on observations from disassembly

[fallenoak]

Depends on https://github.com/wowserhq/blizzardry/pull/94

[timkurvers]

Nice! 👍