btile: foreground tiles

[jorgegv]

Allow for BTILEs that are drawn in front of the hero, i.e., the hero can pass behind them.

• BTILEs with this feature must be of type TT_FOREGOUND (new type).
• They are drawn normally as DECORATION BTILEs but the cells that they occupy are removed from SP1 update list.
• Probably the SP1 update list needs to be reset to again have all cells active also when entering a new screen.