Add drawing toggle and pac ignoring functions

New function `pac.TogglePartDrawing(ent, bool)` allows gamemode creators
to toggle whether parts should be drawn on any entity. A serverside
interface to this function is also provided.

New functions `pac.IgnorePlayer(ply)` and `pac.UnIgnorePlayer(ply)` use
`pac.TogglePartDrawing` to let gamemode creators write interfaces for
clientside disabling of part drawing per-entity.