fl_ice on which movable stones can slide

[alochmann]

In Enigma, ice floor has low friction for actors, but stones are not affected in their movement. There are however numerous examples of games using frictionless ice for puzzle blocks, like certain titles from the Zelda, Pokémon, and Oxyd series. It even is a well established trope: https://tvtropes.org/pmwiki/pmwiki.php/Main/FrictionlessIce

This shows that frictionless ice for stones is a worthwhile addition for Enigma. However, due to compatibility with existing levels, fl_ice cannot be redefined in this way without serious consequences; a new variant has to be defined. I propose to interpret the English term "black ice" literally for this: Create a new floor type resembling fl_ice, but with dark blue or black instead of blue tones, and more sharply defined white structure. The fl_forward code can be reused. Actors should experience the typical friction and adhesion of fl_ice -- after all, these can be redefined by the level anyway.

This new floor type should act like fl_ice in all other aspects, like melting. Therefore, it should be a flavor of fl_ice, say, fl_ice_black.