Add VS2012 update 4 version number.
Use pragma once.
Clean up structs/enums.
Update Changelog.txt.
Update LAV Filters to 0.59.1-31-c635d2d (custom build based on 0.59.1-26-73db2ec).

Important changes:
 - Fix a freeze when changing the bitstreaming settings when paused (ticket #3688).
 - Avoid a possible crash with DXVA2 decoding.
Use conversion constructors to work-around confusing AStyle styling.
Use a bitcheck instead of casting (WIP).

Fixes CID #965944.