added support for fading in and fading out global animations

added support for fading in and fading out global animations

To fade global animations in or out, use the attribute ".fade_mode"
with value "fade_in" or "fade_out" and the attribute ".fade_delay"
with the number of video frames to use for fading in or out. (Note
that this attribute expects frames when used with global animations,
while it expects milliseconds when used with screens and menus.)