Implement a simple benchmarking mode which logs FPS to a file

Very useful to compare performance between two builds, check the impact of
a configuration option, etc. FPS log is stored in User/Logs/fps.txt and is
reset each time you launch a game. Only enabled if you check the "Log FPS
to file" option in your graphics settings.

Could be improved a bit: currently logs only every 1s (so you can't really
see small variations), maybe output more infos to the fps.txt like
average/stddev (but Excel/Libreoffice/Google Docs can compute that easily
too).

Source/Core/VideoCommon/Src/FPSCounter.h

@@ -0,0 +1,28 @@
+// Copyright (C) 2003 Dolphin Project.
+
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, version 2.0.
+
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License 2.0 for more details.
+
+// A copy of the GPL 2.0 should have been included with the program.
+// If not, see http://www.gnu.org/licenses/
+
+// Official SVN repository and contact information can be found at
+// http://code.google.com/p/dolphin-emu/
+
+#ifndef _FPS_COUNTER_H_
+#define _FPS_COUNTER_H_
+
+// Initializes the FPS counter.
+void InitFPSCounter();
+
+// Called when a frame is rendered. Returns the value to be displayed on
+// screen as the FPS counter (updated every second).
+int UpdateFPSCounter();
+
+#endif // _FPS_COUNTER_H_