Add STAT_MATCHSTARTTIME to support new gameclock sync
Add support for gameclock sync stat, STAT_MATCHSTARTTIME

If server supports it, it will send out the game start time to every
player at start of game _and_ at each new connection so that spectators
and players joining mid-game will get synced gameclocks instantly.
Make screenshots work on all resolutions.
Improve chance of download success on non-mvdsv servers