Fix FPS counter and Game Window speed % breaking on pause/unpause

-Add pause state to FPSCounter.
-Add ability to have more than one "OnStateChanged" callback in core.
-Add GetActualEmulationSpeed() to Core. Returns 1 by default. It's used by my input PRs.

Source/Core/VideoCommon/FPSCounter.cpp

@@ -8,6 +8,7 @@
 #include "Common/CommonTypes.h"
 #include "Common/FileUtil.h"
 #include "Common/Timer.h"
+#include "Core/Core.h"
 #include "VideoCommon/FPSCounter.h"
 #include "VideoCommon/VideoConfig.h"
 
@@ -16,6 +17,18 @@ static constexpr u64 FPS_REFRESH_INTERVAL = 250000;
 FPSCounter::FPSCounter()
 {
   m_last_time = Common::Timer::GetTimeUs();
+
+  m_on_state_changed_handle = Core::AddOnStateChangedCallback([this](Core::State state) {
+    if (state == Core::State::Paused)
+      SetPaused(true);
+    else if (state == Core::State::Running)
+      SetPaused(false);
+  });
+}
+
+FPSCounter::~FPSCounter()
+{
+  Core::RemoveOnStateChangedCallback(&m_on_state_changed_handle);
 }
 
 void FPSCounter::LogRenderTimeToFile(u64 val)
@@ -31,8 +44,9 @@ void FPSCounter::LogRenderTimeToFile(u64 val)
 
 void FPSCounter::Update()
 {
-  u64 time = Common::Timer::GetTimeUs();
-  u64 diff = time - m_last_time;
+  const u64 time = Common::Timer::GetTimeUs();
+  const u64 diff = time - m_last_time;
+  m_time_diff_secs = static_cast<double>(diff / 1000000.0);
   if (g_ActiveConfig.bLogRenderTimeToFile)
     LogRenderTimeToFile(diff);
 
@@ -47,3 +61,17 @@ void FPSCounter::Update()
     m_time_since_update = 0;
   }
 }
+
+void FPSCounter::SetPaused(bool paused)
+{
+  if (paused)
+  {
+    m_last_time_pause = Common::Timer::GetTimeUs();
+  }
+  else
+  {
+    const u64 time = Common::Timer::GetTimeUs();
+    const u64 diff = time - m_last_time_pause;
+    m_last_time += diff;
+  }
+}