A simple implementation of the matrix movie background.
All posible letters to choose from is stored in the global_varibles.gd sccript under the constant SYMBOLS.
spawn_time: Time it takes before a new instance of a trail will be spawned.
letter_color: The initial color of the letter.
fall_speed: How fast will the trail fall.
trail_length: How many letters should apear behind the initial letter.
trail_seperation: The distance in pixels between each letter in the same trail.
color_fade: The amount of fade each subsequent letter will recive to their color. Each color chanel(rgb) is stored as a float value between 0 and 1, fade will subtract the amount of color fade from each color channel.
rand_change_time: How long it should take before the letter changes after spawn. if smaller than 0 it wont change at all.