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.cpp

@@ -21,6 +21,7 @@
 #include "video_driver.hpp"
 
 bool _video_hw_accel; ///< Whether to consider hardware accelerated video drivers.
+bool _video_vsync; ///< Whether we should use vsync (only if _video_hw_accel is enabled).
 
 void VideoDriver::GameLoop()
 {