Playermodel legs and other player anim improvements

[Blixibon]

This PR makes the following player model-related changes:

• Support for player legs - Uses a modified version of the built-in meathook camera. Every standard human animation model will have a new KV block defining camera offsets, which can be seen in Improved player animations mapbase-model-src#4 alongside other changes related to this PR. Unique models which don't use HL2 human animations may need to define their own to work properly as legs.
• Improved Animation State
  • Weapon lowering gestures - The playermodel will visibly lower its weapon according to the viewmodel.
  • Better duck timing - The playermodel matches up with the player's actual duck timing better.
  • Player choreo gestures - The playermodel will play gestures from choreographed scenes.
  • Other miscellaneous fixes and improvements, including fixes related to the weapon model not drawing properly in first/third person or emitting muzzle flashes in strange ways.
• Better shadow control - 4 cvars related to the player model separately casting projected texture and RTT shadows in first and third person mode have been added. They are prefixed with cl_player_allow_.

Unique walking animations have also been disabled in favor of slowed running animations for the time being. They can be re-enabled using sv_playeranimstate_use_walk_anims.

The animation state class has also been renamed from "Singleplayer animation state" to "Mapbase animation state" due to having several independent changes which may be carried over to the MP port.

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