Currently, there's one animation for the whole canvas. While it looks fine
with just one screen, it causes windows to move between screens when
switching workspaces. Instead, have a separate animation on each screen,
and sync their progress so that at any given time the progress "fraction"
is the same between all screens. Clip all animations to their screens so
that the windows don't leak to other screens.
If a window is placed between every screen, can end up in multiple
animations, in that case each part is still animated separately.
Fixes https://gitlab.gnome.org/GNOME/gnome-shell/issues/1213https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/605
Simplify the logic a bit. Introduce WorkspaceAnimation class that reparents
the windows from current, surrounding and destination workspaces and manages
them. Expose 'progress' property and have WorkspaceAnimationController animate
it instead of animating everything separately.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/605
