Only do x/y pos Tweens if property is changing b/t moves.

Previously, if an entity with the Slide component was being moved (via
slide()), Tweens for both X and Y position changes would be included,
even though entities can (currently) only move along a single axis at
once. This would sometimes cause some jittering along the axis that the
entity was *not* moving along.  This issue seems to have been alleviated
by restricting Tween creation to the axis of movement.

Signed-off-by: Atom Macumber <amacumber@eagleproductivity.com>