added support for stacked global animations

This extension of global animations adds "stacked animations" that are
created by "stacking" the same global animation multiple times, either
horizontally, vertically, or both, with additional options to use some
empty space between two adjacent stacked animations, as follows:

global.anim_X.stacked_xfactor: <x-factor>
global.anim_X.stacked_yfactor: <y-factor>
global.anim_X.stacked_xoffset: <x-offset>
global.anim_X.stacked_yoffset: <y-offset>

This would stack global animation X <x-factor> times horizontally and
<y-factor> times vertically, while adding empty space between stacked
animations of <x-offset> pixels horizontally and <y-offset> vertically.