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/sdl2_opengl_v.cpp

@@ -70,8 +70,6 @@ const char *VideoDriver_SDL_OpenGL::Start(const StringList &param)
 	SDL_GetWindowSize(this->sdl_window, &w, &h);
 	this->ClientSizeChanged(w, h, true);
 
-	SDL_GL_SetSwapInterval(GetDriverParamBool(param, "vsync") ? 1 : 0);
-
 	return nullptr;
 }
 
@@ -91,6 +89,11 @@ void VideoDriver_SDL_OpenGL::DestroyContext()
 	}
 }
 
+void VideoDriver_SDL_OpenGL::ToggleVsync(bool vsync)
+{
+	SDL_GL_SetSwapInterval(vsync);
+}
+
 const char *VideoDriver_SDL_OpenGL::AllocateContext()
 {
 	SDL_GL_SetAttribute(SDL_GL_RED_SIZE, 8);
@@ -107,6 +110,8 @@ const char *VideoDriver_SDL_OpenGL::AllocateContext()
 	this->gl_context = SDL_GL_CreateContext(this->sdl_window);
 	if (this->gl_context == nullptr) return "SDL2: Can't active GL context";
 
+	ToggleVsync(_video_vsync);
+
 	return OpenGLBackend::Create(&GetOGLProcAddressCallback);
 }