int vga_addmode(int width, int height, int num_cols, int offset, int bytesperpixel )
Adds a mode to the list of modes, with the given parameters.
The function returns the mode number. If such a mode already exists on the list,
its number is returned, and no mode is added.
Adding a mode to the list is not enough in order to use it. There must
also be a timing line that fits that mode. This can be added either as a modeline in the config file,
or with one of the functions