This function adds data for a 2D position (x and y) to the new vertex format being created.
vertex_format_add_position();
N/A
vertex_format_begin();
vertex_format_add_colour();
vertex_format_add_position();
my_format = vertex_format_end();
The above code creates a new vertex format with just colour and position values and then stores the format in the variable my_format.