Update 0.8.1 Changelog

A small hotfix for P2CE:

• Fixed crash loading certain valid textures
• Removed unnecessary campaign-related assets from release
• Added empty ChapterBackgrounds.txt to override Portal 2's
• Added compile tools benchmark scripts
• Don't load mounts.kv if -nousermount or -safemode is passed to the game
• Added brae's get_hammer_content and get_latest_js_types scripts, and ported them to PowerShell
• The convert_bik_to_webm PowerShell script should now work on UNIX platforms
• Update install_hammer script's downloaded dependencies