Make timer_Time() threadsafe

[Philip Taylor]

timer_Time isn't thread-safe, due to t_last.

CLogger::PushRenderMessage uses it (to record the times when warnings/errors are logged), which may be called from a background thread.

If it's hard to fix, we could hack around it by having CLogger use a non-monotonic and/or low-precision timer instead, but it'd be nicer if the function worked.

(Incidentally, t_last+DBL_EPSILON seems wrong - if t_last >= 2.0 then I expect t_last+DBL_EPSILON == t_last. I don't think it'd necessarily be bad to remove that guarantee and expect callers to handle 0 (though still guarantee non-negative).)

[Jan Wassenberg]

(In [8516]) thread-safe timer monotonicity check. fixes #653 (agree that DBL_EPSILON is an unnecessary hack, since division by (t+eps)-(t) will yield a number not much more useful than infinity)