improved smooth movement animation for BD engine (complete rewrite)

improved smooth movement animation for BD engine (complete rewrite)

This is a complete rewrite of the smooth movement animation code for
the BD game and graphics engine. The previous code was a mess, and
still did not cover all cases (like game elements leaving a tile while
another game element is entering the same tile in the same cycle).

The new code only redraws a single playfield tile (without also trying
to redraw the corresponding adjacent tile related to element movement)
by drawing the tile background, the part of the element leaving that
tile and the part of the element entering that tile (without redrawing
the neighboring tile where the movement starts or stops, which is now
redrawn completely independently, allowing for correctly handling all
special cases in a much more clean way).