Feature: allow a toggle to enable/disable vsync

Vsync should be off by default, as for most players it will be
better to play without vsync. Exception exist, mainly people who
play in fullscreen mode.

src/video/video_driver.hpp

@@ -28,6 +28,7 @@ extern std::vector<Dimension> _resolutions;
 extern Dimension _cur_resolution;
 extern bool _rightclick_emulate;
 extern bool _video_hw_accel;
+extern bool _video_vsync;
 
 /** The base of all video drivers. */
 class VideoDriver : public Driver {
@@ -66,6 +67,12 @@ public:
 	 */
 	virtual bool ToggleFullscreen(bool fullscreen) = 0;
 
+	/**
+	 * Change the vsync setting.
+	 * @param vsync The new setting.
+	 */
+	virtual void ToggleVsync(bool vsync) {}
+
 	/**
 	 * Callback invoked after the blitter was changed.
 	 * @return True if no error.