Fix delay before throttling emulation after fast-forward ends

Previously `m_throttle_deadline` was not being updated during
fast-forward, causing the emulator to try to render as many frames as
possible in zero seconds. And when you released fast-forward, the
emulator would continue to fast-forward for an emulated time equal to
the physical time you held the key, waiting for `m_throttle_deadline` to
catch up to the physical time. This was only limited when the "System
can not to keep up with timings!" fallback would keep the target
timebase 100ms before the current time, so the emulator would never
fast-forward more than 100ms emulated time after you released Tab.

Instead update `m_throttle_deadline` to track wall-clock time, so
emulation resumes throttling as usual once fast-forward ends.

Source/Core/Core/CoreTiming.cpp

@@ -373,6 +373,11 @@ void CoreTimingManager::Throttle(const s64 target_cycle)
   const TimePoint min_deadline = time - m_max_fallback;
   const TimePoint max_deadline = time + m_max_fallback;
 
+  if (speed <= 0.0)
+  {
+    m_throttle_deadline = time;
+  }
+
   if (m_throttle_deadline > max_deadline)
   {
     m_throttle_deadline = max_deadline;