Don't sync mode cvars when vid_restart doesn't set a mode

VID_Restart (false) is used to recreate render resources without
touching the window (vsync, FSAA and OIT mode changes). It still called
VID_SyncCvars, which reset vid_width/vid_height/vid_refreshrate/
vid_fullscreen to the current mode, discarding pending selections in
the video menu. Toggling vsync therefore reverted an unapplied
resolution or fullscreen choice.

Author: Novum

Quake/gl_vidsdl.c

@@ -4656,7 +4656,8 @@ void VID_Restart (qboolean set_mode)
 	//
 	// keep cvars in line with actual mode
 	//
-	VID_SyncCvars ();
+	if (set_mode)
+		VID_SyncCvars ();
 
 	//
 	// update mouse grab