Improvements to glFenceSync support

-Now inserts fence after call to SwapBuffers()
-Now waits for synchronization immediately before processing user input
-Fixed a few other issues that can occur with it
Moved latency tester support into separate cvar/option

-Device enumeration occuring every frame has a non-trivial impact on
performance
-As such, latency tester support was moved out of vr_ovr_debug and into
vr_ovr_latencytest
-HMD sensor no longer re-enumerates if it's not connected when vr mode
is enabled