fixed various edge cases for smooth movement animation for BD engine

fixed various edge cases for smooth movement animation for BD engine

The recent complete rewrite of smooth movement animations for the BD
engine still had a number of bugs and edge cases not correctly handled
(or handled at all). These problems have been fixed now, using separate
variables for game elements in the playfield array and drawing elements
that differ from the "real" game elements, to be able to correctly use
game element properties for those game elements that have "looks like"
definitions to use completely different game elements (with different
element properties) just for drawing (like the expandable wall).