~kusabana/vsp-frame-snapshot-fix

Fix for sv_parallel_sendsnapshot crashes in Source Engine 2013 games

The problem

// If enabled, random crashes start to appear in WriteTempEntities, etc. It looks like
// one thread can be in WriteDeltaEntities while another is in WriteTempEntities, and both are
// partying on g_FrameSnapshotManager.m_FrameSnapshots. ...
static ConVar sv_parallel_sendsnapshot( "sv_parallel_sendsnapshot", "0" );

This was later fixed in the CS:GO engine branch by using a CThreadFastMutex for the m_FrameSnapshots list.