This mirrors the logic from calc_frame_time(). When VSync is enabled,
waiting for the previous frame to complete might induce long wait periods,
up to the complete frame time, so we should also handle multiplayer there.
For the other sync modes, the GL calls will simply block, so we can't do
anything about that (except going multithreaded, but that's a totally
different can of worms). Note that even without sync, the SwapBuffers()
call in gr_flip() may also block if VSync is on (and enough frames are
queued up), so the issue is not the GL sync code itself.
SYNC_GL_FENCE_SLEEP is now probably the nicest mode for multiplayer
with VSsync, as it is the only one which has the potential to continue
handling multiplayer packets during the wait for VBlank time.
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