Make getTime() relative to first invocation

src/modules/timer/Timer.cpp

@@ -123,7 +123,7 @@ double Timer::getTimerPeriod()
 	return 0;
 }
 
-double Timer::getTime()
+double Timer::getTimeAbsolute()
 {
 	// The timer period (reciprocal of the frequency.)
 	static const double timerPeriod = getTimerPeriod();
@@ -156,5 +156,11 @@ double Timer::getTime()
 #endif
 }
 
+double Timer::getTime()
+{
+	static const double start = getTimeAbsolute();
+	return getTimeAbsolute() - start;
+}
+
 } // timer
 } // love