Begin a new GFX transaction, which is a sequence of GFX mode changes. The idea behind GFX transactions is to make it possible to activate several different GFX changes at once as a "batch" operation. For example, assume we are running in 320x200 with a 2x scaler (thus using 640x400 pixels in total). Now, we want to switch to 640x400 with the 1x scaler. Without transactions, we have to choose whether we want to first switch the scaler mode, or first to 640x400 mode. In either case, depending on the backend implementation, some ugliness may result. E.g. the window might briefly switch to 320x200 or 1280x800. Using transactions, this can be avoided.